Packaging fails after Upgrading to UE5.6

Hi, I just upgraded my project from UE5.5 to UE5.6 and everything works fine. Only after trying to package my project to a Windows Standalone App I realized something went wrong.

Everytime I try to package, doesn’t matter which configuration, it gives me this Error output:

UATHelper: Packaging (Windows): LogWindows: Error: appError called: Assertion failed: Index >= 0 [File:D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Public\UObject\UObjectArray.h] [Line: 1037]
UATHelper: Packaging (Windows): LogWindows: Error: begin: stack for UAT
UATHelper: Packaging (Windows): LogWindows: Error: === Critical error: ===
UATHelper: Packaging (Windows): LogWindows: Error:
UATHelper: Packaging (Windows): LogWindows: Error: Assertion failed: Index >= 0 [File:D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Public\UObject\UObjectArray.h] [Line: 1037]
UATHelper: Packaging (Windows): LogWindows: Error:
UATHelper: Packaging (Windows): LogWindows: Error:
UATHelper: Packaging (Windows): LogWindows: Error:
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc15b24018 UnrealEditor-Core.dll!FDebug::CheckVerifyFailedImpl2() [D:\build++UE5\Sync\Engine\Source\Runtime\Core\Private\Misc\AssertionMacros.cpp:728]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc0a455324 UnrealEditor-Kismet.dll!UE::Private::GetAllImportedObjects'::2’::FExternalReferenceFinder::operator<<() [D:\build++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:170]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc24d68e9d UnrealEditor-CoreUObject.dll!FArchiveUObject::SerializeObjectPtr() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\Serialization\ArchiveUObject.cpp:92]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc15d0da62 UnrealEditor-Core.dll!FStructuredArchiveSlot::operator<<() [D:\build++UE5\Sync\Engine\Source\Runtime\Core\Private\Serialization\StructuredArchiveSlots.cpp:304]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc25160a1c UnrealEditor-CoreUObject.dll!FObjectProperty::SerializeItem() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\PropertyObject.cpp:180]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc24e399a4 UnrealEditor-CoreUObject.dll!UStruct::SerializeBin() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp:1242]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc24e416ac UnrealEditor-CoreUObject.dll!UScriptStruct::SerializeItem() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp:3365]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc251df9db UnrealEditor-CoreUObject.dll!FStructProperty::SerializeItem() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\PropertyStruct.cpp:170]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc24e399a4 UnrealEditor-CoreUObject.dll!UStruct::SerializeBin() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp:1242]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc00147803 UnrealEditor-BlueprintGraph.dll!UK2Node_FunctionEntry::Serialize() [D:\build++UE5\Sync\Engine\Source\Editor\BlueprintGraph\Private\K2Node_FunctionEntry.cpp:280]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc0a4dd86a UnrealEditor-Kismet.dll!UE::Private::GetAllImportedObjects() [D:\build++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:223]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc0a4dbcf2 UnrealEditor-Kismet.dll!UE::Private::GetAllDependencies() [D:\build++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:287]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc0a4c677b UnrealEditor-Kismet.dll!UE::Private::GenerateBlueprintDependencies() [D:\build++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:589]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc0a535528 UnrealEditor-Kismet.dll!UE::Private::BlueprintDependencies::RecordCookDependencies() [D:\build++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:691]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc0cc8646b UnrealEditor-Engine.dll!UBlueprint::OnCookEvent() [D:\build++UE5\Sync\Engine\Source\Runtime\Engine\Private\Blueprint.cpp:959]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc252141fc UnrealEditor-CoreUObject.dll!UE::SavePackageUtilities::CallCookEventPlatformCookDependencies() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage\SavePackageUtilities.cpp:701]
UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc251d9b99 UnrealEditor-CoreUObject.dll!anonymous namespace'::RoutePresave() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage2.cpp:288] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc251da21a UnrealEditor-CoreUObject.dll!UPackage::Save2() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage2.cpp:3791] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc251da7a1 UnrealEditor-CoreUObject.dll!UPackage::Save() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage.cpp:20] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc11cd91c5 UnrealEditor-UnrealEd.dll!UEditorEngine::Save() [D:\build\++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\EditorEngine.cpp:4525] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc119f9898 UnrealEditor-UnrealEd.dll!UCookOnTheFlyServer::SaveCookedPackage() [D:\build\++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\Cooker\CookSavePackage.cpp:126] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc12afc5bc UnrealEditor-UnrealEd.dll!UCookOnTheFlyServer::PumpRuntimeSaves() [D:\build\++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\CookOnTheFlyServer.cpp:5053] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc12b0ba24 UnrealEditor-UnrealEd.dll!UCookOnTheFlyServer::TickMainCookLoop() [D:\build\++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\CookOnTheFlyServer.cpp:1528] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc12b0b1ac UnrealEditor-UnrealEd.dll!UCookOnTheFlyServer::TickCookByTheBook() [D:\build\++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\CookOnTheFlyServer.cpp:1408] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc11779940 UnrealEditor-UnrealEd.dll!UCookCommandlet::RunCookByTheBookCook() [D:\build\++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\Commandlets\CookCommandlet.cpp:598] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc1173a69a UnrealEditor-UnrealEd.dll!UCookCommandlet::CookByTheBook() [D:\build\++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\Commandlets\CookCommandlet.cpp:550] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc11756c83 UnrealEditor-UnrealEd.dll!UCookCommandlet::Main() [D:\build\++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\Commandlets\CookCommandlet.cpp:269] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ff6e4248fa0 UnrealEditor-Cmd.exe!FEngineLoop::PreInitPostStartupScreen() [D:\build\++UE5\Sync\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp:3860] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ff6e423e39c UnrealEditor-Cmd.exe!GuardedMain() [D:\build\++UE5\Sync\Engine\Source\Runtime\Launch\Private\Launch.cpp:143] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ff6e423e6ba UnrealEditor-Cmd.exe!GuardedMainWrapper() [D:\build\++UE5\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:128] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ff6e424209e UnrealEditor-Cmd.exe!LaunchWindowsStartup() [D:\build\++UE5\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:282] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ff6e4254e44 UnrealEditor-Cmd.exe!WinMain() [D:\build\++UE5\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:339] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ff6e4257466 UnrealEditor-Cmd.exe!__scrt_common_main_seh() [D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288] UATHelper: Packaging (Windows): LogWindows: Error: [Callstack] 0x00007ffc8020e8d7 KERNEL32.DLL!UnknownFunction [] UATHelper: Packaging (Windows): LogWindows: Error: UATHelper: Packaging (Windows): LogWindows: Error: end: stack for UAT PackagingResults: Error: appError called: Assertion failed: Index >= 0 [File:D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Public\UObject\UObjectArray.h] [Line: 1037] PackagingResults: Error: begin: stack for UAT PackagingResults: Error: === Critical error: === PackagingResults: Error: Assertion failed: Index >= 0 [File:D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Public\UObject\UObjectArray.h] [Line: 1037] PackagingResults: Error: [Callstack] 0x00007ffc15b24018 UnrealEditor-Core.dll!FDebug::CheckVerifyFailedImpl2() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\Misc\AssertionMacros.cpp:728] PackagingResults: Error: [Callstack] 0x00007ffc0a455324 UnrealEditor-Kismet.dll!UE::Private::GetAllImportedObjects’::2'::FExternalReferenceFinder::operator<<() [D:\build\++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:170] PackagingResults: Error: [Callstack] 0x00007ffc24d68e9d UnrealEditor-CoreUObject.dll!FArchiveUObject::SerializeObjectPtr() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\Serialization\ArchiveUObject.cpp:92] PackagingResults: Error: [Callstack] 0x00007ffc15d0da62 UnrealEditor-Core.dll!FStructuredArchiveSlot::operator<<() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\Serialization\StructuredArchiveSlots.cpp:304] PackagingResults: Error: [Callstack] 0x00007ffc25160a1c UnrealEditor-CoreUObject.dll!FObjectProperty::SerializeItem() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\PropertyObject.cpp:180] PackagingResults: Error: [Callstack] 0x00007ffc24e399a4 UnrealEditor-CoreUObject.dll!UStruct::SerializeBin() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp:1242] PackagingResults: Error: [Callstack] 0x00007ffc24e416ac UnrealEditor-CoreUObject.dll!UScriptStruct::SerializeItem() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp:3365] PackagingResults: Error: [Callstack] 0x00007ffc251df9db UnrealEditor-CoreUObject.dll!FStructProperty::SerializeItem() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\PropertyStruct.cpp:170] PackagingResults: Error: [Callstack] 0x00007ffc24e399a4 UnrealEditor-CoreUObject.dll!UStruct::SerializeBin() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp:1242] PackagingResults: Error: [Callstack] 0x00007ffc00147803 UnrealEditor-BlueprintGraph.dll!UK2Node_FunctionEntry::Serialize() [D:\build\++UE5\Sync\Engine\Source\Editor\BlueprintGraph\Private\K2Node_FunctionEntry.cpp:280] PackagingResults: Error: [Callstack] 0x00007ffc0a4dd86a UnrealEditor-Kismet.dll!UE::Private::GetAllImportedObjects() [D:\build\++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:223] PackagingResults: Error: [Callstack] 0x00007ffc0a4dbcf2 UnrealEditor-Kismet.dll!UE::Private::GetAllDependencies() [D:\build\++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:287] PackagingResults: Error: [Callstack] 0x00007ffc0a4c677b UnrealEditor-Kismet.dll!UE::Private::GenerateBlueprintDependencies() [D:\build\++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:589] PackagingResults: Error: [Callstack] 0x00007ffc0a535528 UnrealEditor-Kismet.dll!UE::Private::BlueprintDependencies::RecordCookDependencies() [D:\build\++UE5\Sync\Engine\Source\Editor\Kismet\Private\Blueprints\BlueprintDependencies.cpp:691] PackagingResults: Error: [Callstack] 0x00007ffc0cc8646b UnrealEditor-Engine.dll!UBlueprint::OnCookEvent() [D:\build\++UE5\Sync\Engine\Source\Runtime\Engine\Private\Blueprint.cpp:959] PackagingResults: Error: [Callstack] 0x00007ffc252141fc UnrealEditor-CoreUObject.dll!UE::SavePackageUtilities::CallCookEventPlatformCookDependencies() [D:\build\++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage\SavePackageUtilities.cpp:701] PackagingResults: Error: [Callstack] 0x00007ffc251d9b99 UnrealEditor-CoreUObject.dll!anonymous namespace’::RoutePresave() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage2.cpp:288]
PackagingResults: Error: [Callstack] 0x00007ffc251da21a UnrealEditor-CoreUObject.dll!UPackage::Save2() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage2.cpp:3791]
PackagingResults: Error: [Callstack] 0x00007ffc251da7a1 UnrealEditor-CoreUObject.dll!UPackage::Save() [D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage.cpp:20]
PackagingResults: Error: [Callstack] 0x00007ffc11cd91c5 UnrealEditor-UnrealEd.dll!UEditorEngine::Save() [D:\build++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\EditorEngine.cpp:4525]
PackagingResults: Error: [Callstack] 0x00007ffc119f9898 UnrealEditor-UnrealEd.dll!UCookOnTheFlyServer::SaveCookedPackage() [D:\build++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\Cooker\CookSavePackage.cpp:126]
PackagingResults: Error: [Callstack] 0x00007ffc12afc5bc UnrealEditor-UnrealEd.dll!UCookOnTheFlyServer::PumpRuntimeSaves() [D:\build++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\CookOnTheFlyServer.cpp:5053]
PackagingResults: Error: [Callstack] 0x00007ffc12b0ba24 UnrealEditor-UnrealEd.dll!UCookOnTheFlyServer::TickMainCookLoop() [D:\build++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\CookOnTheFlyServer.cpp:1528]
PackagingResults: Error: [Callstack] 0x00007ffc12b0b1ac UnrealEditor-UnrealEd.dll!UCookOnTheFlyServer::TickCookByTheBook() [D:\build++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\CookOnTheFlyServer.cpp:1408]
PackagingResults: Error: [Callstack] 0x00007ffc11779940 UnrealEditor-UnrealEd.dll!UCookCommandlet::RunCookByTheBookCook() [D:\build++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\Commandlets\CookCommandlet.cpp:598]
PackagingResults: Error: [Callstack] 0x00007ffc1173a69a UnrealEditor-UnrealEd.dll!UCookCommandlet::CookByTheBook() [D:\build++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\Commandlets\CookCommandlet.cpp:550]
PackagingResults: Error: [Callstack] 0x00007ffc11756c83 UnrealEditor-UnrealEd.dll!UCookCommandlet::Main() [D:\build++UE5\Sync\Engine\Source\Editor\UnrealEd\Private\Commandlets\CookCommandlet.cpp:269]
PackagingResults: Error: [Callstack] 0x00007ff6e4248fa0 UnrealEditor-Cmd.exe!FEngineLoop::PreInitPostStartupScreen() [D:\build++UE5\Sync\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp:3860]
PackagingResults: Error: [Callstack] 0x00007ff6e423e39c UnrealEditor-Cmd.exe!GuardedMain() [D:\build++UE5\Sync\Engine\Source\Runtime\Launch\Private\Launch.cpp:143]
PackagingResults: Error: [Callstack] 0x00007ff6e423e6ba UnrealEditor-Cmd.exe!GuardedMainWrapper() [D:\build++UE5\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:128]
PackagingResults: Error: [Callstack] 0x00007ff6e424209e UnrealEditor-Cmd.exe!LaunchWindowsStartup() [D:\build++UE5\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:282]
PackagingResults: Error: [Callstack] 0x00007ff6e4254e44 UnrealEditor-Cmd.exe!WinMain() [D:\build++UE5\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:339]
PackagingResults: Error: [Callstack] 0x00007ff6e4257466 UnrealEditor-Cmd.exe!__scrt_common_main_seh() [D:\a_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
PackagingResults: Error: [Callstack] 0x00007ffc8020e8d7 KERNEL32.DLL!UnknownFunction
PackagingResults: Error: end: stack for UAT

Does anyone have an idea why that happens? I didn’t really change anything except upgrading from UE5.5 to UE5.6.

Hello there @ArvurRobin!

Checking your scenario around the community, “Assertion failed: Index >= 0 [File:D:\build++UE5\Sync\Engine\Source\Runtime\CoreUObject\Public\UObject\UObjectArray.h] [Line: 1037]” aims at a conflict with an element or reference inside your project being corrupt or invalid, something entirely possible due migrating to a new UE.

The first step would be to identify this element, most likely a blueprint. Use “Fix Up Redirectors” and “Validate Assets” from the Content Browser, then check the Reference Viewer to locate any broken links. After that, go to File > Refresh All Nodes, and save the project.

If the issue persists, then there might be old data causing problems. Please perform a cache clear, via deleting folders Saved, DerivedDataCache, and Intermediate.

Alternatively, test the pack process in a new, blank 5.6 project, to rule out if the issue is general, or tied to the original project.

Thanks for your reply, brs-sebascova.

I already tried “Update Redirector References”, “Refresh Blueprints”, “Validate Assets in Folder” and “Resave All” with all the Content / Blueprint Class Assets in my project and plugins in my project. I checked the Output Log every time and there was not a single issue shown.

I also tried creating a new project and moving my content there but the very same error is shown.

I tried disabling all plugins (except the few I really need) but still the Error is thrown.

I checked all the logs to find any hint on what Asset is giving me the Error, but unfortunately there’s nothing.

I spent the last few hours opening every single Blueprint Asset and checking if there is any Warning or Error anywhere.
As my project is mostly written in C++ I even checked if there is any problem with my C++ Code, but I don’t find anything.

The very same project worked completely fine in UE5.5 but now in UE5.6 there is this one weird issue.
Everything works completely fine in Editor, but packaging always leads to that Error.
I tried packaging on a different machine as well, but same happens there.

Any ideas are welcome! :slight_smile:

Hello again!

Thank you for your update. Considering you have covered all steps, and no clear error shows up in blueprints and code, yet the issue remains, the problem is most likely tied to an under the hood interaction between your project and 5.6.

Since this version of the engine has just released, it is still prone to unexpected behaviours. I would suggest sticking to 5.5 for the moment, until a hotfix is deployed, then you can test again.

Ofcourse that is a possibility, but not the way I want to go. There has to be a way to figure out which Asset is the one that leads to the Error.

Are there any other tools you know of to figure that out? This is not only about my problem right now, but in general. I’m sure there are a lot of developers out there running into a similar issue at some point and knowing where to look for is important.

So: Do you know any other tools, workflows or steps to take a deeper look into the cooking process to understand what causes this issue?
Thanks in advance!

Hi @brs-sebascova
I figured out a bit more.
I now know in which of the Plugins I created the Error happens.
It’s one or multiple Widget Blueprints. I have a couple of them for the User Interface.
Some reference between them (Some are created as sort of a Template and I embed them into others while others are used as ListView Entries) is the issue.
I’m left with 24 Blueprint Widgets and somewhere in there there’s the culprit(-s).

So now that I’ve specified it a lot more:
Are there any helpful tools / steps / workflows to better understand where the reference error happens?

So far I see:
Deleting all 24 = No Error when cooking
Deleting all the references in between them = No Error when cooking
Going through them and deleting just one or two references = ERROR

Ofcourse I could go on and just search for the needle in the haystack, but this is the Unreal Engine. I’m sure there is some kind of built-in tool / community plugin / workflow for this that I just don’t know of.
Thanks in advance for your help!

Hello again!

Glad to hear about your progress here, you are in the right track.

As for built-in solutions in UE, you are already working with Reference Viewer. Opening all 24 widgets with it should result in a graph with all assets and their references. The workflow here would be to check the most common or central references, and look for any broken links, circular dependencies, or paths to non-existing objects.

Another option would be to use Asset Audit, by selecting all 24 widgets, then right click > Asset Actions > Audit Assets, to check for assets’ integrity.

But at the end of the day, this will most likely end in good ol’ trial and error. I would suggest removing all references, splitting the widgets in groups of 8, and re-add the references for one group at a time, then pack. With each pass, the numbers of suspects will be reduced, until you find the cultrip.

I’ve created a simple scene with a floor, a cube and a hdri backdrop then I packed the scene…everything was just fine but when I add a directional light I cannot pack it anymore, I get an error. Then I decided to delete the light and pack again but failed. Maybe is something with the lighting system

I had the same error and it was caused by the Text3D plugin not working on 5.6. So, if you used that plugin on any BP or Level, you need to replace them even if they aren’t showing the Text3D element inside.