(this is a duplicate of #2005, but I am creating a new issue for visibility reasons)

An unknown interaction between Carpet 1.4.147 for Minecraft 1.21 and 1.21.1 and polymorph-fabric-1.0.7+1.21.1 causes a java.lang.ClassCastException during the encoding of Carpet's server-to-client packet.
This issue does not occur on integrated servers, only when a client attempts to connect to a dedicated server.

Here is a log featuring the stack trace from a test server with only Fabric API, Carpet and Polymorph installed: latest.log file

Also linking the issue on Polymorph's repo for completeness' sake.