Basically that query is failing because it looks like the person in question isn't assigned to any group. It builds your forum list off your groups, and for some reason this person isn't even getting into the anonymous groups, his groups are just null resulting in a bad query.
Is this on threadsdev? Maybe need to do a query and see if there is anyone with no groups.
Select U_Username from w3t_Users where U_Groups = '' or U_Groups = '-"
That should cover it unless their profile is really screwed up, in which case:
Select U_Username from w3t_Users where U_Groups = '' or U_Groups = '-" or U_Groups = '--'
<img src="https://www.ubbcentral.com/boards/images/graemlins/laugh.gif" alt="" />
