Said mod: https://www.curseforge.com/minecraft/mc-mods/mobs-main-menu

How to reproduce:
Install both newest version of MMM and Requiem.
Result:
When game finishes loading, screen flashes several times, and no mob entity dummy renders in menu; only player dummy does. When loading the world no entity/block entity renders either, hand and player entity does not too.
Error:
Appears at the end of the log, after generating assets
[16:34:46] [Thread-501/INFO]: OpenGL debug message, id=1283, source=API, type=ERROR, severity=HIGH, message=GL_STACK_OVERFLOW error generated. Matrix stack overflow.
Fix:
Specific cause of the issue is unknown for now.

Looks like MMM hasn't been updated for 2 MC versions, so I'll be closing this issue. If it gets updated and the issue persists, please reopen !

Thanks for the report ! Really not obvious that one.

I will also add link here to issue ticket on MMM repo:
GabrielOlvH/MobsMainMenu#3