https://vborg.vbsupport.ru/showthread.php?t=228692
You are the second one reporting this.
http://www.vbulletin.com/forum/project.php?issueid=6099
Maybe it is a character encoding issue?
--------------- Added [DATE]1259137365[/DATE] at [TIME]1259137365[/TIME] ---------------
Just checked your link. The username is not empty, it contains characters not supported by your OS; I do not have multilingual support installed, but I suppose it is something from an asian charset.