Player(s) can't connect

Hi! So, I'm running a Pixelmon generations server for my friends and as far as I know everything is set up right since I'm able to connect, but my friend keeps getting the error "Internal Exception: io.netty.handler.codec.EncoderException: java.lang.OutOfMemoryError". I told them they should download Pixelmon through the Technic Launcher, which is what I'm assuming they did. Their problem is that they can't connect to my server or any other server. I've looked up this problem but I can't find a solid answer, mainly just 'change your username, it's too long' but neither or our users are over 10 characters. I also haven't had any other person connect to my server so I don't know if it's just them having the problem. I'd love a solution ASAP and I can try to provide any additional info, thanks!!