If you have the mirror shard equipped and are attacked by a guardian, it causes the server to lag out and eventually crash due to a stack overflow error.

I'm using the FTB Unstable 1.16.3 pack, and I've replicated this with a clean install of the pack and only the mirror shard equipped and no other piece of equipment at all.

Steps to replicate:
equip mirror shard
spawn in guardian
Get attacked
Lag leading to server crash.