This problem is fixed in Description of the Outlook 2010 hotfix package (Outlook-x-none.msp): December 10, 2013
After installing Office 2010 SP2 (or an earlier update), Outlook 2010 users started noticing Event ID 27, "Calendar Folder property is missing" warnings in the Event viewer's Application log. While annoying, it's not harmful and should only be generated when Outlook is first started, one event for each calendar in the profile.
Microsoft discovered this warning is triggered for users who have a PST in their profile, including PSTs that were created by connecting to a SharePoint site, subscribing to Internet Calendars, as well as used by POP3 and IMAP accounts and AutoArchive.
Microsoft's workaround? Remove the PST files. Seriously.
To work around this problem, PSTs can be removed from the profile. To remove PSTs from your Outlook profile, follow these steps:
1. Open Microsoft Outlook 2010.
2. Click File, and then click Account Settings.
3. Click the Data Files tab.
4. Select the data file that you want to remove, and then click Remove.
Removing PSTs from your profile will not remove the PSTs from your system. Removing some PSTs may result in loss of functionality in Outlook. For example, removing the SharePoint Lists PST will remove connections to SharePoint sites.
"Removing some PSTs may result in loss of functionality in Outlook." "may" reduce functionality? I think removing any PST from a profile is highly likely to reduce functionality in Outlook, especially if it belongs to a POP3 or IMAP account.
Personally, I'll live with the annoying events and keep my SharePoint connections, while I wait for a real fix. It's easy enough to create a filter in the event log that hides Event 27 and they'll age off.
To create a filter, open the Event viewer and right-click on Application log and choose Filter Current Log.
Type -27 in the Includes/Excludes field in the middle of the dialog and click Ok. This will hide Event 27 but show all other events.