Impossible to build lightmass since UE 5.2

[RedYggdrasil]

Hello everyone,
Since at least Unreal 5.2 (I’ve updated to 5.2.1 today to test if it was fixed, apparently not). I Can’t build a level’s lights, it constantly result in a crash.

Also, just in case, I use the engine by source, not epic game launcher.

I tried both on my project and on new “first person” project to build a simple level with just one box set as static and the default light already present ( a sky sphere, directional light ect, the “Basic” setup).
It “Build” by default (as there are no static object, there is no job. But as soon as I put an actor/Mesh with a mobility of “static”, I get this crash :

<None> === Lightmass crashed: ===
Assertion failed: CurrentMaterialElement.Material [File:C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Lighting\LightingMesh.cpp] [Line: 146] 
Failed to import material with Hash 5413E0D38F6DF9B7509BB558DECE2468DD77487E
0x00007ff6b0fd8388 UnrealLightmass.exe!Lightmass::FStaticLightingMesh::Import() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Lighting\LightingMesh.cpp:146]
0x00007ff6b103a73d UnrealLightmass.exe!Lightmass::FStaticMeshStaticLightingMesh::Import() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Lighting\StaticMesh.cpp:299]
0x00007ff6b0fa75b4 UnrealLightmass.exe!Lightmass::FLightmassImporter::ImportObjectArray<Lightmass::FStaticMeshStaticLightingMesh,TMap<FGuid,Lightmass::FStaticMeshStaticLightingMesh *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<FGuid,Lightmass::FStaticMeshStaticLightingMesh *,0> > >() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\ImportExport\Importer.h:155]
0x00007ff6b0fd6923 UnrealLightmass.exe!Lightmass::FScene::Import() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\ImportExport\LightmassScene.cpp:207]
0x00007ff6b0fbb39f UnrealLightmass.exe!Lightmass::BuildStaticLighting() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\CPUSolver\CPUSolver.cpp:179]
0x00007ff6b0fe0a25 UnrealLightmass.exe!Lightmass::LightmassMain() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Launch\UnrealLightmass.cpp:269]
0x00007ff6b1001547 UnrealLightmass.exe!main() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Launch\UnrealLightmass.cpp:551]
0x00007ff6b107056c UnrealLightmass.exe!__scrt_common_main_seh() [D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
0x00007ffe909f26ad KERNEL32.DLL!UnknownFunction []
0x00007ffe925ea9f8 ntdll.dll!UnknownFunction []



0x00007ff6b0fd8388 UnrealLightmass.exe!Lightmass::FStaticLightingMesh::Import() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Lighting\LightingMesh.cpp:146]
0x00007ff6b103a73d UnrealLightmass.exe!Lightmass::FStaticMeshStaticLightingMesh::Import() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Lighting\StaticMesh.cpp:299]
0x00007ff6b0fa75b4 UnrealLightmass.exe!Lightmass::FLightmassImporter::ImportObjectArray<Lightmass::FStaticMeshStaticLightingMesh,TMap<FGuid,Lightmass::FStaticMeshStaticLightingMesh *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<FGuid,Lightmass::FStaticMeshStaticLightingMesh *,0> > >() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\ImportExport\Importer.h:155]
0x00007ff6b0fd6923 UnrealLightmass.exe!Lightmass::FScene::Import() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\ImportExport\LightmassScene.cpp:207]
0x00007ff6b0fbb39f UnrealLightmass.exe!Lightmass::BuildStaticLighting() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\CPUSolver\CPUSolver.cpp:179]
0x00007ff6b0fe0a25 UnrealLightmass.exe!Lightmass::LightmassMain() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Launch\UnrealLightmass.cpp:269]
0x00007ff6b1001547 UnrealLightmass.exe!main() [C:\UE_B\UnrealEngine\Engine\Source\Programs\UnrealLightmass\Private\Launch\UnrealLightmass.cpp:551]
0x00007ff6b107056c UnrealLightmass.exe!__scrt_common_main_seh() [D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
0x00007ffe909f26ad KERNEL32.DLL!UnknownFunction []
0x00007ffe925ea9f8 ntdll.dll!UnknownFunction []

With the highlight seeming to be “Failed to import material with Hash […]”.

Maybe I fail to do something important, maybe Basic level is not compatible with baked light anymore ? I don’t know what I am missing there, I’ve updated Unreal, Visual Studio, My GPU drivers, and I don’t know where to go from there.

Thanks for reading me,

RedYggdrasil.

[RedYggdrasil]

Okay, it seems than rebuilding the UnrealLightmass project inside UE5.sln fixed the problem.
I found that out after trying with another computer where a UE popup explicitly asked me to do so
“Unreal Lightmass executable is outdated. Recompile Unreal Lightmass project with Development configuration in Visual Studio.”

So apparently I was just indeed missing a basic thing.

However, I though than updating UE5 was just doing that :
→ Pulling New Release
→ Running Setup.bat
→ Running GenerateProjectFile.bat
→ Building UE5 project inside UE5.sln
And voila

But apparently it is not enough since I had to explicitly rebuild UnrealLightmass independently afterward for it to work. Am I missing some basic & important steps there ?