Error was like this:
=== Lightmass crashed: ===
Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:D:\Build\++UE4+Licensee\Sync\Engine\Source\Runtime\Core\Public\Containers/Array.h] [Line: 674]
Array index out of bounds: 3223 from an array of size 3223
0x00007fffbfc27c57 UnrealLightmass-Core.dll!UnknownFunction []
0x00007ff7f135b568 UnrealLightmass.exe!DispatchCheckVerify >() []
0x00007ff7f129ca2c UnrealLightmass.exe!Lightmass::FEmbreeGeometry::FEmbreeGeometry() []
0x00007ff7f12a5fbb UnrealLightmass.exe!Lightmass::FEmbreeAggregateMesh::AddMesh() []
0x00007ff7f12f67b5 UnrealLightmass.exe!Lightmass::FStaticLightingSystem::FStaticLightingSystem() []
0x00007ff7f12aa529 UnrealLightmass.exe!Lightmass::BuildStaticLighting() []
0x00007ff7f12d10d7 UnrealLightmass.exe!Lightmass::LightmassMain() []
0x00007ff7f12ef8d3 UnrealLightmass.exe!main() []
0x00007ff7f1356c64 UnrealLightmass.exe!__scrt_common_main_seh() []
0x00007ff864957bd4 KERNEL32.DLL!UnknownFunction []
0x00007ff86680ce51 ntdll.dll!UnknownFunction []
0x00007ff863b1a719 KERNELBASE.dll!UnknownFunction []
0x00007fffbfe62db6 UnrealLightmass-Core.dll!UnknownFunction []
0x00007fffbfe66197 UnrealLightmass-Core.dll!UnknownFunction []
0x00007fffbfc93b2d UnrealLightmass-Core.dll!UnknownFunction []
0x00007fffbfc25935 UnrealLightmass-Core.dll!UnknownFunction []
0x00007fffbfc27c90 UnrealLightmass-Core.dll!UnknownFunction []
0x00007ff7f135b568 UnrealLightmass.exe!DispatchCheckVerify >() []
0x00007ff7f129ca2c UnrealLightmass.exe!Lightmass::FEmbreeGeometry::FEmbreeGeometry() []
0x00007ff7f12a5fbb UnrealLightmass.exe!Lightmass::FEmbreeAggregateMesh::AddMesh() []
0x00007ff7f12f67b5 UnrealLightmass.exe!Lightmass::FStaticLightingSystem::FStaticLightingSystem() []
0x00007ff7f12aa529 UnrealLightmass.exe!Lightmass::BuildStaticLighting() []
0x00007ff7f12d10d7 UnrealLightmass.exe!Lightmass::LightmassMain() []
0x00007ff7f12ef8d3 UnrealLightmass.exe!main() []
0x00007ff7f1356c64 UnrealLightmass.exe!__scrt_common_main_seh() []
0x00007ff864957bd4 KERNEL32.DLL!UnknownFunction []
0x00007ff86680ce51 ntdll.dll!UnknownFunction []