Well, I am using the same 3.1 build 121 and my pwd is 11 characters so I don't think that's it. When you go to the Trillian Connections > Manage My Connections screen, do you see the account RuppWorld1 via AIM displayed with a login box? Is it in this login box when you click connect you get the error? Did you receive and reply to the confirmation email when you registered the screen name? Maybe you should just register a new name and see what happens, it took me all of 3 minutes to register, get the email and login via Trillian.
Of course by now you probably already talked to this person by phone anyway, hehe.