I'm not sure I will be able to help, but the first thing I would check is Console One. In the menu under Groupwise Utilities --> User Move status I would make sure that the user you are having problems with fully moved to the new PostOffice. If he has completely moved, I would remove the User in BES and readd him. Then activate him again, I would make sure you back up his contacts before you proceed. Other than those two things I can't think of anything else to check.
Now if RIM would release BES 4.1.5 or 4.1.6 for Groupwise I would be ecstatic.