We are using Exchange at our site and a person has this error here, as well. If it were an Exchange issue, everyone at a corporate networked site would have the error, not just one person, as is the case here... unless you are suggesting it is limited to just that person's Exchange profile.... But we found that not to be the case.
I found some more information on this topic - for those with Experts-Exchange accounts, you may want to look at this:
http://www.experts-exchange.com/Net.../Exchange_Server/Q_21891363.html?qid=21891363
Otherwise, the solution accepted there can be found here:
http://josheinstein.com/journal/archive/2004/09/26/216.aspx
Basically, (summarizing) this is a result of the Add-in for the Out-of-Office Assistant not successfully loading at some point for the user, Outlook prompting the user to disable it because it didn't load correctly, and the user doing so.
Long story short, either log in as another user, set up your own Outlook profile, and hope the add-in loads correctly (do NOT disable it again if prompted, or you'll have to use yet another user's account to attempt again - once it's disabled for a user, it has to load correctly for the registry entry to reset back to being a "good" Add-in!) and then log in as them and see if it comes back, or do a Detect and Repair in Outlook under their account and say to "Reset my settings", which you'll have to do if it does ask you to disable it again under the other user profile.
Also, it may be listed as a "Disabled Item" in the section you can click into on the "About Microsoft Office Outlook" link on the Help menu.
If the Detect and Repair doesn't work, and it's not in the Disabled Items, or is and can't be re-enabled, an Office uninstall and re-install may be required.
Tom
