Tentatively marking this SOLVED. The problem did not reappear when I configured iCal to access the user's Zimbra account from a different login on the same machine. So I decided it must be some corrupt data on the computer, not on Zimbra.
Solution: turn off iCal setup in Zimbra control panel, delete CalDAV account in iCal. Then move the following folders to the Trash (or some temporary location):
~/Library/Calendars
~/Library/Application Support/iCal
~/Library/Caches/com.apple.iCal
To be on the safe side I'd also move:
~/Library/Preferences/iCalExternalSync.plist
~/Library/Preferences/com.apple.iCal.plist
~/Library/Preferences/com.apple.iCal.helper.plist
~/Library/Preferences/ByHost/com.apple.iCal.helper.<arbitrary string>.plist
~/Library/Preferences/comp.apple.iCal.plist
~/Library/Preferences/com.apple.iCal.AlarmScheduler.plist
Then re-enable CalDAV setup in Zimbra control panel and confirm that account is set up by launching iCal and checking prefs.