I'd like to jump in here because we had exactly the same problem. I submitted it to support (complete with an .ics dump of the event and the actual invite that created the problem, case #00022960) and it has kind of languished with support despite it being a real time bomb. A single invite took down our Dean's calendar and the calendars of all of the people who shared his calendar.
Let me describe how we were able to track down the offending event and just what caused it. I can't promise that this exactly matches your situation, but perhaps it will help.
First off, the way we found the event was to essentially page through the user's calendar, month by month until we had that error message pop up. Once you do that, you can then switch to week view and perhaps finally day view to finally determine which event is causing the problem (you'll get the error message when you hover over the right one). In our case, the user had a good guess as to which event had recently come in and that made narrowing the search down easier.
I suppose one could also find it by exporting the user's calendar and attempting to import it again to another scratch user's calendar. It should throw a better error that way. If you really want to get gnarly, I believe I have the necessary SQL (provided by support the first time I reported the problem) to find all calendar events with attachments.
Here's how the problem was created for us: the event in question was caused by an invite sent to this user from Lotus Notes that had an attachment. The attachment had a comma in its name, but Lotus notes did not properly quote it on the outgoing mail. The specific header in question said:
According to RFC2045, certain characters have to be quoted or in quotes if they are to be used as a parameter value in a MIME header:
Content-Type: application/octet-stream; name=SBI Meeting Notes - May 19, 2008.doc
As you can see above, Notes clearly blows it in this regard. Somehow Zimbra incorporated the event from the invite but created a broken attachment and this makes everything go blooey. At my request the main Notes admin here submitted this as a bug to IBM. He has been told that it has been escalated to their development group for a subsequent fix. I also asked the person who sent the invite to change her file naming scheme (as they say in vaudeville, if your arm hurts when you move it a certain way, don't move it that way).
tspecials := "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" / <"> "/" / "[" / "]" / "?" / "=" ; Must be in quoted-string, ; to use within parameter values
I assert that the Zimbra folks need to fix their parsing code so that it doesn't create a poison event. The person who was handling our support case (not naming names here) said he could not get the invite to be accepted via zmlmtpinject because it threw parse errors and he was going to file a request to have their parser handle this better. And that's where it got left with me despite my nervous pleas.
One alternate theory I have is this event got accepted by Mail.app/iCal into the user's calendar and then injected into the Zimbra calendar by the iSync Connector (i.e. it didn't get into the system in a way that zmlmtpinject could replicate, it came in a back door). No one from Zimbra has told me if they have tested that possibility even though I mentioned this theory to them.
In any case, I assert this situation is bad news and actually needs to be hunted down with a little more vigor. I feel your pain and would be glad to help if I can in any way.