Crash report: https://paste.dimdev.org/vafavejego.mccrash

Thanks to VanillaFix for catching this crash. Had it not been caught by VanillaFix, he would have had to restart a Minecraft client with over 200 mods (takes around 30 minutes).

He tells me that pressing any key other than Shift (obvious) crashes the game. (Even Escape)

Interesting, i'll look into it

That actually happened twice. The first time I can't remember what key I hit. The second time I pressed a mere "W".

Might be a conflict with MultiHotbars... It only works on his first hotbar.

I don't know what you have going on with the first hotbar, but no other mod has this issue.