Have been running a small server with some friends and there seems to be an outdated version of Neutrino Packet Tweaker within the latest 1.21.5 polymer bundle which is incorrectly causing players to be kicked due to packet issues.

The used packet tweaker version (v0.6.0-pre.1+1.21.2-pre3) is the latest one and it's compatible with 1.21.5. Your packet errors are probably caused by something else

Yes I'm sorry! Was reading the logs too quickly. It was a "Server Backpacks!" mod that broke things.