I would say 95% of use account related problems of that nature should be in the user's home library.
That being said, one final check is to give the user a clean home folder. Easiest way to do that is to enable root and login as root. Browse to the user's home and make a subfolder "backup", and drag and drop everything else into there. Then login as that user.
If that won't let them login, then there's a file or entry somewhere in a system area relating to that user by account ID. From there, the easiest fix is to make a new user, and replace the new user's home with the broken user's home. Change ownership and call it done.
