All our 10.8.x Mac users are experiencing multiple Calendar issues. It has gotten worse in the last week, although there have been no updates on the server or Mac end that we are aware of that would cause these issues. They are syncing to our email server using CalDAV.

The mail server is Zimbra (version 8.0.2). We are not seeing anything in our server logs that would indicate a problem. All users outside of those using Mac 10.8.x are fine (over 500 other users running Outlook, webmail only, or ZD). Also, our users have used the same setup for many years and have never experienced anything like this. Most have been on 10.8.x for many months now.

Scenario 1: Organizer sends out calendar invite to users. Users never receive invite and server logs never show invite hitting the server.

Scenario 2: Organizer sends out calendar invite to users. First user accepts invite and it disappears from the organizers calendar.

Scenario 3: One organizer reported that their Calendar app sent out many invites just today for invites created the past two months. Our server shows the invites were generated by "CalendarAgent". He says he did nothing special this morning on his Mac. He just started receiving emails and phone calls from invitees asking why he was sending invites out for meetings in the past.

This is all very random. Our IT staff has sent cal invites between users having the problem and it works each time. Also, the system logs show no issues that appears to be related. The most common repeating calendar error is "Could not parse the ics contain the default alarms. Using none as the default alarms."

Apple support has not been any help at all. They blame our mail setup. Any suggestions on where else to look for what might be causing this?