Navision 2009 calendar for date entry opens up and closes immediately

Navision 2009 R2 [version US Dynamics Nav 6.0 R2] (on Citrix client) user has a problem on which every time selection of the "Posted Date" [ or Due Date ] column is selected to bring up the calendar for date entry, it opens up and closes immediately.   A date can be entered by typing the date in the correct format although.   The system is a DELL Windows 7 Pro SP1 desktop, up to date on all recommended updates (even MS KB2830477).  This only happens on this particular workstation.  Other workstations (desktop or laptops) with the same NAV credentials do not have this problem.   Any suggestions?   Thanks.