Im also getting this array bounds error on a custom source 4.21 build.
Im using the source (hybrid) install engine modification zip, tried full rebuild of ue4 and lightmass and clearing caches
[SPOILER]10:05:36 PM: Exception handled in main, calling appHandleCriticalError
10:05:36 PM: LogLightmass: Error: === Critical error: ===
10:05:36 PM:
10:05:36 PM: Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:d:\ue4source\ue4wvxgi\unrealengine\engine\source\runtime\core\public\Containers/Array.h] [Line: 611]
10:05:36 PM: Array index out of bounds: 518 from an array of size 0
10:05:36 PM: 0x00007ff9ac3ba388 KERNELBASE.dll!UnknownFunction ]
10:05:36 PM: 0x00007ff91a3cab4a UnrealLightmass-Core.dll!UnknownFunction ]
10:05:36 PM: 0x00007ff91a233547 UnrealLightmass-Core.dll!UnknownFunction ]
10:05:36 PM: 0x00007ff91a1781a9 UnrealLightmass-Core.dll!UnknownFunction ]
10:05:36 PM: 0x00007ff674c973d9 UnrealLightmass.exe!Lightmass::FStaticLightingSystem::FinalizeSurfaceCacheTextureMapping() [d:\ue4source\ue4wvxgi\unrealengine\engine\source\programs\unreallightmass\private\lighting exturemapping.cpp:431]
10:05:36 PM:
10:05:36 PM: 0x00007ff674c97b03 UnrealLightmass.exe!Lightmass::FStaticLightingSystem::FinalizeSurfaceCacheThreadLoop() [d:\ue4source\ue4wvxgi\unrealengine\engine\source\programs\unreallightmass\private\lighting exturemapping.cpp:256]
10:05:36 PM:
10:05:36 PM: 0x00007ff674c7d0bd UnrealLightmass.exe!Lightmass::FMappingProcessingThreadRunnable::Run() [d:\ue4source\ue4wvxgi\unrealengine\engine\source\programs\unreallightmass\private\lighting\lightingsystem.cpp:1389]
10:05:36 PM: 0x00007ff91a3ca380 UnrealLightmass-Core.dll!UnknownFunction ]
10:05:36 PM: 0x00007ff91a3ba951 UnrealLightmass-Core.dll!UnknownFunction ]
10:05:36 PM: 0x00007ff9af813034 KERNEL32.DLL!UnknownFunction ]
10:05:36 PM: 0x00007ff9afe31471 ntdll.dll!UnknownFunction ]
10:05:36 PM: === Critical error: ===
10:05:36 PM: [/SPOILER]