Describe the bug
I have Paper-1.21.6 installed and when I started the server it returned an error posted in screenshots below.

To Reproduce
Steps to reproduce the behavior:
Install ProtocolLib and Paper1.21.6 on your server and start it up
Screenshots

Version Info
ProtocolLib 5.3.0
Paper-1.21.6
Can't dump because plugin is disabled

can confirm, having the same issue. for whatever reason using it in paper gradle runServer seems to work fine though.

same issue happens after swapping to 1.21.5 spigot

This is fixed in the latest dev builds. Find them here: https://ci.dmulloy2.net/job/ProtocolLib/

This is fixed in the latest dev builds. Find them here: https://ci.dmulloy2.net/job/ProtocolLib/

The plugin itself is fixed on later builds, however I am having classpath issues while trying to update my gradle dependency