I'm running a vanilla compatible fabric server with polylib installed. Joining on a vanilla client works fine and joining on a forge client works fine. Joining on a fabric client (without polylib installed on the client) causes the following error:

The server is running Fabric 0.15.11, here's the full modlist (with FTBBackups2 being the only mod with polylib as a requisite):

This issue only appeared after updating the server to 1.21, running the same mods on 1.20.4 didn't have this issue.

Fixed in latest build