I get this crash log on startup with Immersive Engineering:

The game crashed whilst rendering overlay
Error: java.lang.IllegalStateException: Failed to load description for particle strawgolem:fly
Exit Code: -1

https://pastebin.com/5tcXXrHR

I see someone else mentioned the fly particle issue, what "older" version of forge is recommended for 1.16.3?

Mod needs to be patched for 1.16.3, best to use 1.16.2 for now.

Although I can't seem to reproduce this crash with the mod alone in the numerous times I've tried...

I'm looking into it

Sorry about that!

EDIT: Try with forge 34.1.0, and also without Optifine, and let me know if the issue still occurs?

Turns out this doesn't seem to be my fault, but IE's
BluSunrize/ImmersiveEngineering#4410