You don't mention which version of Outlook you are using, but I just found a bunch of warnings for my 2010 version that mentions "Calendar Folder property is missing (or empty)". You might check the Event Viewer and look in the Warnings section.
It appears my situation is being caused by a recent update. Repairing the install did not help. I do not know if this might be effecting your install, but it is a coincidence.