happens when you open JEI

Forge version forge-1.16.3-34.1.25
Mekanism version 1.16.3-10.0.15.441
Minecraft 1.16.3
jei-1.16.3-7.6.0.48

https://pastebin.com/kgr7YRtn

Can you try it with the latest Forge version(34.1.32)?

Is that a full crash report? It doesn't show that you have Mekanism or JEI installed which it should. Also do you have optifine or another mod like that installed and if so does it happen without it?

Unable to reproduce with latest forge (34.1.32), mek (10.0.15), and JEI (7.6.0.49). I only installed the mek core, not the other modules, so maybe that's it? Will try that

EDIT: Works fine with additions, generators, and tools as well.
EDIT 2: You have optifine installed. Remove it.