Outlook 2007: Error opening Shared Calendars
 

My company is running Office 2007 on Vista Business and using Exchange 2003.
Many users are reporting an intermitant error when accessing Shared Calendars.
This does not appear to be specific to any particular calendars.

The messaging interface has returned an unknown error. If the problem
persists, restart Outlook.

Closing Outlook and re-opening resolves the problem.
All Windows and Office updates have been applied, and Office Diagnostics
does not report any problems.

Any help would be appreciated.