I'm having the exact same issue. I have tried Centos Zimbra Open Source 5.0.7 and I have tried Mac OS X 10.4 client with Network Editions 5.0.6 and just upgraded to 5.0.7 and am having the same issue.
My client machines are running 10.5.3 and 10.5.4 with their respective versions of iCal and no luck at all. Tried the sync connector and manually entering the server info and nothing helps.
I can view their calendars, but I cannot edit any of them. I get an error every time. I've tried everything I can think of and am not getting much help from admins or employees.
I figure if I keep posting this issue then maybe it will be fixed in 5.0.8!