Ah....
UPDATE w3t_Users
SET U_Groups = '-3-5-'
WHERE U_Number = 'xxx'
Have you done much hacking? If you haven't done much to the admin section, I would try uploading a clean copy of the /admin directory... then see if the group problem is fixed. It would seem to me that an old script, without the get_input function of 6.2 might be kicking around.