May be a "corrupt" calendar folder. I had to remove a clients
Calendar and recreate it because of this.
Look on Sue Mosher's Slipstick site for Ffolder Utility:
Ffolder Utility: Utility to rename or delete "special"
Outlook folders such as the Calendar, Tasks, Inbox, and
Contacts. VB source code included.
http://www.slipstick.com/addins/gallery/index.htm#ffolder
Look on technet for the information on how to rebuild the
Calendar system folder with the "/ResetFolders" switch:
OL2000: Additional Command-Line Switches
http://support.microsoft.com/default.aspx?scid=kb;en-us;197180
However, if you had information in the Calendar folder you may
want to open Outlook using the ".../Safe" switch first and try saving
your calendar info into another folder first.