First, some background on this client:

* they have ~50 users, all using Mac computers on a pure Mac network

* the shared calendar is from a local exchange server on the network using "user name and password" authentication. 

* the directory service logs in with their Exchange account

* there are no delegates

* there are no digital certificates used

* there are no local accounts.  All users log in via a network login.

* only one user is experiencing this error

* I have rebuilt the users Outlook database multiple times.  Doing so will correct the issue for a day, then the shared calendar disappears again.

* I have reinstalled OS X from a USB thumb drive.  This had no effect.

* I have removed and reinstalled MS Office 2011 for Mac. No effect.

* I have not erased and recreated the users network account nor have I recreated the Exchange account yet (this is my last resort)

Basically the user states that only their calendar is affected.  Their e-mails both sending and receiving work fine, even after the calendar disappears.  They state that they will be using Outlook, leave it open and use a different program (of which they have nothing different installed from their co-workers) or just be creating/reading e-mails and when they check their calendar, the company/shared calendar has vanished.  

Rebuilding the Outlook Database on the default ID will NOT correct this issue.  Finding an older "backup" ID, setting that to default and then rebuilding it allows the user to see their calendar and the Office Reminder window.

I am at a loss here.  Any help is greatly appreciated.