The join thing on the website just reads from a list of pre-written messages. If you attempt to join a server that is full, then join one not full, it will show the message you are receiving. This is a minor bug, I highly doubt it will be fixed.
If you still can't join, try clearing your cookies (I like chocolate chip, but Google Chrome doesn't :P), sometimes cookies can do that. Try rebooting your PC if that doesn't fix it. Also visit http://www.roblox.com/download to re-install ROBLOX to your computer.
When everything above fails, email [email protected] with browser (e.g. Google Chrome), operating system (e.g. Windows 8.1), username, and any other helpful information.
~Santey Claws