It is some glitch on the Yahoo chat room servers. I saw a good number of people posting the exact same problem, so I decided to test it out. The problem occurred on some Yahoo IDs and not others. So what could be an option is to try a different Yahoo ID. Another thing that worked sometimes, was to sign out of Yahoo Messenger, sign back in and try to join a room. Sometimes I had to repeat this a few times, before I was successful.
Hopefully the glitch will be cleared up by tomorrow.