I had the same problem once.
http://www.vbulletin.com/forum/showthread.php?t=250538
Quote:
It looks like your autoincrement values are off. You can use phpmyadmin to examine the database and set the appropriate autoincrement value for the user table. Or we can do this for you if you submit a trouble ticket with a URL and login to phpmyadmin:
http://members.vbulletin.com/members...ontactform.php
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I sent in a ticket and they fix it very quickly so if you want to save the headache I suggest just submit a ticket.