If Apple iCal ceased to work with your CalDAV server following Apple's upgrade of the OS, I recommend reporting your problems to Apple.
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http://radar.apple.com/>
Also consider reporting to the
Zimbra Bugzilla. I see some issues relating to Snow Leopard but at a glance, not the issue(s) that are described in this thread.
Comparable issues with iCal 4.0 (1362) and CalDAV service of Chandler Hub (an instance of Chandler Server a.k.a. Cosmo) are discussed at
iCal 4.0 (1362) Mac OS X 10.6.1 interop issues with Chandler Hub.