I've recently been making a multiplayer server on aternos.org ,
and this server is modded and didn't run into any issues untill i ran it on minecraft and it said this ,
"io.netty.handler.codec.decoderexception java.io.ioexception bad packet id 64"
I've seen PurplePomes post about this except he had a paid for server, I don't know how to breach into the server console with this website.
I'm not sure what other information you need in order to fix this problem, i'll be willing to answer more questions if it can help !