This behavior sounds like the username may have been a reserved word for the suhosin security module; once you submitted the page the suhosin module would have flagged the stream as a 403 error. Try looking at your webserver logs for the 403 error and reply back the contents of that line (note, we only need that one line to diagnose further).
The course of action, as I doubt your host is interested in removing keywords from their suhosin configuration, would be to go into PHPMyAdmin and edit the users information and edit out the offending data (ONLY do this if you cannot get into your forums because of a Suhosin issue, you can completely destroy your database if you're unsure of what you're doing).
HUGE NOTE, we CANNOT SUPPORT database edits outside of the UBB.threads
I have a test script that you can use which will tell you if your server is running suhosin, it is the UBB.threads System Requirements Test