io.netty DecoderException kicking me off my minecraft server

nine times out of ten that's one mod not matching between you and the server, a different version of one jar or a client-only mod the server doesn't have. open logs/latest.log right after the kick and look at the lines just above the exception, the mod name is usually in there. if the lists genuinely match, it can be a packet bigger than vanilla allows, and there are small mods that raise that limit.