Version: configured-fabric-1.21.1-2.6.0.jar
I get an error that says io.netty.handler.codec.EncoderException: Failed to encode packet 'clientbound/minecraft:custom_payload' (configured:session_data) when trying to join server, both client and server use same Minecraft and Fabric versions, and the mentioned mod version.

I also got this issue whenever I tried joining a server I set up. I fixed it by removing the Configured mod from the server mods folder.