Could it be the server my opponent was using
Your opponent's server is the same we all use - fibs ^_^
Maybe his/her client is a different one - but that doesn't have an effect on you.
Usually your client send the command "join" to fibs without your help. Either it is a bug in your client (I don't think so) or your client has sent the command but it has never reached fibs ... So you have to send the command again and everything's fine. Nothing to worry about at all.