Crash when opening level in 4.27

We have recently upgraded to 4.27 from 4.26.2.

Now when opening one specific level in the editor, the engine crashes with the following:

Unhandled Exception: EXCEPTION_ACCESS_VIOLATION reading address 0x0000000000000308

UE4Editor_UnrealEd!UEditorEngine::Map_Load() [D:\Build\++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\EditorServer.cpp:2746]
UE4Editor_UnrealEd!UEditorEngine::HandleMapCommand() [D:\Build\++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\EditorServer.cpp:6210]
UE4Editor_UnrealEd!UEditorEngine::Exec() [D:\Build\++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\EditorServer.cpp:5711]
UE4Editor_UnrealEd!UUnrealEdEngine::Exec() [D:\Build\++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\UnrealEdSrv.cpp:697]
UE4Editor_UnrealEd!FEditorFileUtils::LoadMap() [D:\Build\++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\FileHelpers.cpp:2554]
UE4Editor_AssetTools!FAssetTypeActions_World::OpenAssetEditor() [D:\Build\++UE4\Sync\Engine\Source\Developer\AssetTools\Private\AssetTypeActions\AssetTypeActions_World.cpp:20]
UE4Editor_UnrealEd!UAssetEditorSubsystem::OpenEditorForAsset() [D:\Build\++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\Subsystems\AssetEditorSubsystem.cpp:363]
UE4Editor_UnrealEd!UAssetEditorSubsystem::OpenEditorForAssets_Advanced() [D:\Build\++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\Subsystems\AssetEditorSubsystem.cpp:437]
UE4Editor_UnrealEd!UAssetEditorSubsystem::OpenEditorForAssets() [D:\Build\++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\Subsystems\AssetEditorSubsystem.cpp:553]
UE4Editor_ContentBrowserAssetDataSource!ContentBrowserAssetData::EditOrPreviewAssetFileItems() [D:\Build\++UE4\Sync\Engine\Plugins\Editor\ContentBrowser\ContentBrowserAssetDataSource\Source\ContentBrowserAssetDataSource\Private\ContentBrowserAssetDataCore.cpp:298]
UE4Editor_ContentBrowserAssetDataSource!ContentBrowserAssetData::EditOrPreviewItems() [D:\Build\++UE4\Sync\Engine\Plugins\Editor\ContentBrowser\ContentBrowserAssetDataSource\Source\ContentBrowserAssetDataSource\Private\ContentBrowserAssetDataCore.cpp:318]
UE4Editor_ContentBrowserAssetDataSource!UContentBrowserAssetDataSource::BulkEditItems() [D:\Build\++UE4\Sync\Engine\Plugins\Editor\ContentBrowser\ContentBrowserAssetDataSource\Source\ContentBrowserAssetDataSource\Private\ContentBrowserAssetDataSource.cpp:1030]
UE4Editor_ContentBrowser!SContentBrowser::OnItemsActivated() [D:\Build\++UE4\Sync\Engine\Source\Editor\ContentBrowser\Private\SContentBrowser.cpp:2281]
UE4Editor_ContentBrowser!TBaseSPMethodDelegateInstance<0,SContentBrowser,0,void __cdecl(TArrayView<FContentBrowserItem const ,int>,enum EAssetTypeActivationMethod::Type),FDefaultDelegateUserPolicy>::Execute() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Core\Public\Delegates\DelegateInstancesImpl.h:290]
UE4Editor_ContentBrowser!SAssetView::OnListMouseButtonDoubleClick() [D:\Build\++UE4\Sync\Engine\Source\Editor\ContentBrowser\Private\SAssetView.cpp:3548]
UE4Editor_ContentBrowser!TBaseSPMethodDelegateInstance<0,SAssetView,0,void __cdecl(TSharedPtr<FAssetViewItem,0>),FDefaultDelegateUserPolicy>::ExecuteIfSafe() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Core\Public\Delegates\DelegateInstancesImpl.h:307]
UE4Editor_ContentBrowser!TDelegate<void __cdecl(TSharedPtr<FAssetViewItem,0>),FDefaultDelegateUserPolicy>::ExecuteIfBound<void,0>() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Core\Public\Delegates\DelegateSignatureImpl.inl:599]
UE4Editor_ContentBrowser!SListView<TSharedPtr<FAssetViewItem,0> >::Private_OnItemDoubleClicked() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Slate\Public\Widgets\Views\SListView.h:1074]
UE4Editor_ContentBrowser!STableRow<TSharedPtr<FAssetViewItem,0> >::OnMouseButtonDoubleClick() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Slate\Public\Widgets\Views\STableRow.h:461]
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FBubblePolicy,FPointerEvent,<lambda_2b5148752c2a272000c47b69ff14a42e> >() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:378]
UE4Editor_Slate!FSlateApplication::RoutePointerDoubleClickEvent() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:5297]
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonDoubleClickEvent() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:5284]
UE4Editor_Slate!FSlateApplication::OnMouseDoubleClick() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:5247]
UE4Editor_ApplicationCore!FWindowsApplication::ProcessDeferredMessage() [D:\Build\++UE4\Sync\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsApplication.cpp:2209]
UE4Editor_ApplicationCore!FWindowsApplication::DeferMessage() [D:\Build\++UE4\Sync\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsApplication.cpp:2698]
UE4Editor_ApplicationCore!FWindowsApplication::ProcessMessage() [D:\Build\++UE4\Sync\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsApplication.cpp:1881]
UE4Editor_ApplicationCore!FWindowsApplication::AppWndProc() [D:\Build\++UE4\Sync\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsApplication.cpp:905]
user32
user32
UE4Editor_ApplicationCore!FWindowsPlatformApplicationMisc::PumpMessages() [D:\Build\++UE4\Sync\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsPlatformApplicationMisc.cpp:125]
UE4Editor!FEngineLoop::Tick() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp:4851]
UE4Editor!GuardedMain() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Launch\Private\Launch.cpp:178]
UE4Editor!GuardedMainWrapper() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:137]
UE4Editor!LaunchWindowsStartup() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:273]
UE4Editor!WinMain() [D:\Build\++UE4\Sync\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:320]
UE4Editor!__scrt_common_main_seh() [d:\agent\_work\5\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
kernel32
ntdll

Any idea what to do to get the level to work again?

Look for errors in your project logs folder (..\[Your Peoject]\Saved\Logs), which gives you a better idea of what caused it.

Thanks for the answer Bernard, although the log does unfortunately not help much.

It says this:
[2021.09.24-07.29.13:820][708]LoadErrors: New page: Loading map: Gym_Combat_Test.umap
before throwing the callstack mentioned above.

But I don’t see any reference to our code or assets that should cause the issue.

Errors do not necessarily cause a crash; try to look for Critical Error; it mentions what caused it.
…also, try Fix up Redirecotors and Validate Assets in your Content folder.

Open you map as blueprint. I used “find in blueprints” for found node in blueprint of map.

Check number of nested Child Actors in Actor on you crash map. Number of nested Child must be less 5. ActorsFor opening map you can remove null actors from UWorld->GetLevels() Level->Actors in c++ and save asset. Map crash when trying get brush from Actors[1] and select NULL