Calendar can't decline the event "(null)" on the exchange server
How can I make this error message go away for good?
iMac, OS X Mountain Lion
Alternatively, if you happen to know which event is causing the problem, you can resolve it by deleting the event from the Exchange calendar either through Outlook Webmail, or using the Outlook client.
If you can't determine the event that caused the problem, deleting and re-adding will almost always work.
On a couple of occasions, I have had to delete the calendar data files in /Users/<name>/Library/Calendars. You must then re-add all your calendars.