We are having the same problem with the list of shared calendars disappearing. This happens about once a month. One of the network people thinks it may have something to do with Office updates that are pushed out about that often. But I cannot prove it.
We use Outlook 2007 Exchange client. We do not use Share Point Services.
On another site, I read that an Outlook file gets corrupted and has to be rebuilt. But it seems like it is just as much trouble rebuilding the file as it is to open the shared calendars again so they go back on the list.
Here's the quote from the internet:
Outlook ate my shortcuts!
http://blog.palehorse.net/2007/09/26/outlook-ate-my-favorite-shortcuts/
"You can navigate to the folder that Outlook stores settings in. In Windows XP it defaults to:
C:\Documents and Settings\<username>\Application Data\Microsoft\Outlook
In this folder you will find a file called Outlook.xml. Very innocuous sounding, don’t you think? This is the file causing all of the woes! You should exit Outlook completely then rename the file (never delete outright, bad form my good Pan!). Restart Outlook and you will notice all of your favorites and calendars are gone. Reset them to the way you would like, exit Outlook again and restart. *Poof* they are back!
Now the problem appears to be some sort of corruption to this file. The odd thing is, the file is only saved when you exit Outlook. I have confirmed this by watching the file system when starting Outlook with the file already gone. It does not reappear until I exit. My best guess is that if a corrupt file is loaded it messes something up within Outlook itself. No error that I could find is generated; however, the file is never saved again.
After some more poking around (it was easier to search for Outlook.xml than for the symptom itself) it seems that the general consensus is that this file becomes corrupt more often than you would imagine. If you’ve had this problem once, chances are pretty good that you will once again find your Favorite Shortcuts not being saved so keep these instructions handy!
An observation
As a side note, it was interesting to view the “corrupt” version of Outlook.xml and a “good” version of the file. They are very similar, but it would appear that one section called <userDefined> is the one that does not get saved after the file has went down the drain."
