OWA users receive a - This operation can't be performed - error when saving appointments or responding to meeting requests, how can I fix this?

There are two possible causes for the “This operation can't be performed” error:
  1. The SystemAttendantDN registry key may be missing.

    Solution: To add the SystemAttendantDN registry key, refer to Microsoft     KB 3104440.
  2. The value of the user object attribute legacyExchangeDN is incorrect. 

    Solution: Follow Microsoft     KB     556073 to correct the legacyExchangeDN attribute value for affected users.  The Global Address Book should be rebuilt after updating the attribute.

    NOTE: The Outlook AutoComplete function may continue to use the incorrect legacyExchangeDN value; add the “incorrect” value to the affected user’s proxyAddresses attribute to ensure email delivery.