Some user from our modpack reported this crash, and looks like there is some interaction with toolbet.
He said this: "I just wanted to get my linking tool back from my tool belt. After restarting my game it worked great. "

https://mclo.gs/bl6YcEz#L15221

Huh, I'll take a look, but if it worked after restarting it may be one of those situations that are hard to debug. I hope not.

Hm... I will take a look soon.

Do you have a crash log for it?

It is the exact same as on original post

I reproduced this bug, just try to place a sophisticated backpack in your belt to crash.