Could not find any similar issues on the list, so let me try to explain what happens in our up-to-date Zimbra with the AJAX-Webclient on Firefox:
An appointment I made a few days ago constantly moves in time and can't be deleted. That means, every time I update the calendar, the appointment starts at the now time. It does not show up in HTML-Calendar nor in the CalDAV, only in the AJAX-client, and I can't delete it there, too: pressing delete results in a message "Do you want to inform the organizer" and pressing Yes nothing happens.
I did not do anything special with the appointment. It was a single time event with two other Zimbra users invited (their appointments are still OK).
Any clue what this might be and how to remove that traveling appointment?