We noticed for a number of our hosting users, and for our own accounts, that Mountain Lion breaks the ability for the Mac Calendar application to create any new appointments or edit any existing appointments.

We opened a Support Case with Zimbra, who were able to reproduce the issue and opened a bug report on our behalf.

The bug report is here: Bug 76450 – Unable to sync iCal using CalDAV on Mountina Lion

If you are impacted by this bug, please vote for it to help Zimbra prioritize their development efforts.

Hope that helps,
Mark

P.S. Thank you to Nikhil Sahasrabudhe at Zimbra for taking the time to work on our support case and reproduce this issue.