I'm not sure exactly what causes the issue, but it has something to do with falling tree entities. I haven't seen this kind of crash before, so I assume it has something to do with the new RenderLib release. I could not reproduce this crash, and I do not know why.

RenderLib: 1.1.0
Forge: 14.23.5.2860
Minecraft: 1.12.2
Dynamic Trees: 0.9.29

The crash happened twice on the client, generating two separate crashlogs.
Simplified Log File, only contains things leading up to the crashes: https://paste.ee/p/aAO2V
Full Log File, with multiple hours of things that don't matter: https://zerobin.net/?69cf163f3dc9efa9#7vPnh9m8R8AcBsAnGGlY5/VNobPGFvamCBNeEkiSRVE=
(Used zerobin due to 2MB size, lot of unnecessary noise that doesn't concern the crash itself)

If you need anything else from me, please let me know.

Looks like some mod which modifies the LightUtil class is the problem but there is no indication that RenderLib is the cause here.

Closing this due to inactivity and no sign that RenderLib is the cause of the crash.