I am trying to access my calendar with Snow Leopard's iCal. I can read
it well, but any modification attempt ends with:

You canít change events or to do items in the "Calendar" calendar.

Only the calendar administrator and any delegates with permission can make changes to read-only calendars. To make changes to the calendar, contact the calendar administrator.

This was after upgrade from Tiger. I upgraded the connector to latest
version from Zimbra Support portal (Zimbra iSync Connector 5.0.18 GA for Mac OS X, version: 5.0.17_GA_2957). I deleted the iCal account and recreated using Zimbra preference pane. No change here.

I am modifying my own primary calendar, not a shared one, so I definitely should have permissions.