I've recently made my first mod just as an experiment, to use on a server with some friends. The mod works wonderfully when I test it on IntelliJ and on singleplayer, but when adding it to the server it doesn't let anyone join and shows this error on the client:
Internal Exception: io.netty.handler.codec.DecoderException: io.netty.handler,codec.Encoder.Exception: java.io.UTFDataFormatException: malformed input around byte 1
When I used any mod other than mine the error didn't appear.
This is the server debug log (it took a while to create a new world cause it's not