Citrix xenapp client screen blanks out when user click on another window

I recently upgraded our Citrix Srver to Xenapp 4.5 on a Windows 2003 server.  Since the upgrade, whenever our clients on the Internet who are logged in to Citrix click any window on their computer outside of Citrix, the client seems to lose focus.  When they click back on the Citrix window, usually all they see is the ouside edges of the window (meaning a square that they can see their desktop or whatever app through).  If they mouse-over the start button in the citrix window, or items on the task bar, the screen will re-draw the area of the screen under the cursor.

Any thoughts?