Start with the first section that refers to Reset Sync History, then navigate to the section for your operating system Mac OS X 10.6.8. Test syncing after each step. If the issue persists see the section regarding syncing calendars:
For issues with syncing calendars
First, you should back up your calendars: Back up your calendars by choosing File > Export > Calendar Archive in OS X Mountain Lion or File > Export > iCal Archive in OS X Lion or Mac OS X v10.6. In Mac OS X v10.5, you can back up your iCal data by selecting File > Back up iCal.
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