When joining through a velocity proxy the server goes into a join loop and than crashes.

It seems like the proxy can't encode a packet related to pehkui. Here is the error from the proxy log:

And here is the cause (it's on the end of the previous image, I just wanted to make it easier by sending the separated cause):

Looks like you'll need to add CrossStitch to your Fabric server in order for it to support Pehkui's custom argument types through Velocity.