Since UseBB 0.5, some important changes have happened to the member system. From now on, a username can only contain alphanumeric characters (a-zA-Z0-9), "_" and "-". Any other characters have been stripped out of the username (spaces will be transformed to "_"). If no characters were left after stripping, the username was set to "user". If any duplicate usernames were created during this process, the duplicate username was followed by a 2, the next one by a 3, and so on.
In short, from now on you should log in with your username without the non-alphanumeric characters. Most people will not discover any problems, but if you do, please be aware about this change.