Yes, as Rick mentioned see if the member has the access rights box unchecked, this could be the source of your problem (unchecking that box is equivalent to banning a user).

If that doesn't help then you can always become that user to see if the account works. If he/she still can't log in then ask them to clear their cookies using this link http://www.yourforumsite.com/ubbthreads/mycookies.php