Error MSG: Unable to display the folder. The folder cannot be found.

I have a single user who appears to have constant issues with their Outlook profile. They can access several shared calendars on our SBS but not their assistants. I have read on this issue dating back to SP1 installs but I recently installed SP3 on their computer and continue to have this issue. Is it an corrupt OST file on the user's part or their assistant's?

Any help would be HUGELY appreciated. This has been going on for weeks.