Which version of Zimbra are you running?
Is there any information in mailbox.log on the server? Also, could you do a trace of the HTTP traffic?
I suspect that this relates to some bugs which have recently been fixed where Zimbra believes that the ORGANIZER has changed (In this instance from nothing to a real value)
When I first looked at the code, I noticed that it didn't have an ORGANIZER and assumed that would be the problem (but it worked for me anyway - the ORGANIZER was introduced with the value of the Calendar owner).
You could try always having an ORGANIZER, even when there are not ATTENDEEs
Lead Engineer - Server
Zimbra | Community & Collaboration