I have not installed Zimbra but have tried to play around with the demo. When I went to the monthly calendar view and double clicked on a date, a new appointment window popped up. Yet I noticed it the background hanged. After I entered the appointment ("OK" or "Cancelled" it), I was faced with a hanged calendar. I have to click on the browser's "back" button to get it to response again. However, when I clicked the "back" button, I was back to the mail screen but it looks like to me that it was thinking I was entering the Zimbra all over again. I checked the calendar afterward and the new appointment was properly added. I tried to do the whole sequence of the above again and again, and it looks like that the above "hang" is repeatable.