I just fixed this error:
=== Lightmass crashed: ===
Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:D:\build++UE5\Sync\Engine\Source\Runtime\Core\Public\Containers\Array.h] [Line: 752]
Array index out of bounds: 699 from an array of size 699
0x00007ff637d49af3 UnrealLightmass.exe!Lightmass::FStaticMeshLOD::GetIndex()
0x00007ff637d4af9d UnrealLightmass.exe!Lightmass::FStaticMeshStaticLightingMesh::GetTriangleIndices()
0x00007ff637cbb3c9 UnrealLightmass.exe!Lightmass::FEmbreeGeometry::FEmbreeGeometry()
0x00007ff637cc516a UnrealLightmass.exe!Lightmass::FEmbreeAggregateMesh::AddMesh()
0x00007ff637d1a8f0 UnrealLightmass.exe!Lightmass::FStaticLightingSystem::FStaticLightingSystem()
0x00007ff637ccbd71 UnrealLightmass.exe!Lightmass::BuildStaticLighting()
0x00007ff637cf10eb UnrealLightmass.exe!Lightmass::LightmassMain()
0x00007ff637d13410 UnrealLightmass.exe!main()
0x00007ff637d816cc UnrealLightmass.exe!scrt_common_main_seh()
0x00007ff82b9826ad KERNEL32.DLL!UnknownFunction
0x00007ff82c6caa68 ntdll.dll!UnknownFunction
0x00007ff637d49af3 UnrealLightmass.exe!Lightmass::FStaticMeshLOD::GetIndex()
0x00007ff637d4af9d UnrealLightmass.exe!Lightmass::FStaticMeshStaticLightingMesh::GetTriangleIndices()
0x00007ff637cbb3c9 UnrealLightmass.exe!Lightmass::FEmbreeGeometry::FEmbreeGeometry()
0x00007ff637cc516a UnrealLightmass.exe!Lightmass::FEmbreeAggregateMesh::AddMesh()
0x00007ff637d1a8f0 UnrealLightmass.exe!Lightmass::FStaticLightingSystem::FStaticLightingSystem()
0x00007ff637ccbd71 UnrealLightmass.exe!Lightmass::BuildStaticLighting()
0x00007ff637cf10eb UnrealLightmass.exe!Lightmass::LightmassMain()
0x00007ff637d13410 UnrealLightmass.exe!main()
0x00007ff637d816cc UnrealLightmass.exe!scrt_common_main_seh()
0x00007ff82b9826ad KERNEL32.DLL!UnknownFunction
0x00007ff82c6caa68 ntdll.dll!UnknownFunction
If you get this error, back up the project and try deleting the models in your level one by one to see which one is causing the error.