Describe the bug
A player with flippers in a backpack crashed the server every time they joined.

To Reproduce
I'm not sure how to reproduce. We have had a few random artifacts crashes, but they're usually resolved after a reboot. In this case it kept occurring.

https://paste.alloffabric.com/esaxorofog.yaml

Hmm, I'm not sure if this belongs here or with CCA.

It's most likely an issue with CCA. It's hard to tell what's happening here since I can't reproduce this. The exception in the crash log you provided should be the cause of an earlier exception. Unless you have a full log that has that initial exception in it (which it might not) then I can't do much about it unfortunately. It could be some compatibility issue, in this case with loading player data from that backpack mod.