Starting last week we've noticed that none of our users can view any events on shared Zimbra calendars in iCal. The CalDAV account shows the calendar that is shared, but none of it's events. And if you try to create an event on a shared calendar, you get the following error in iCal:
Request Error
This is not a valid location for this request
This is happening regardless of whether the calendar is shared Read Only or with Manager privileges. This problem was just noticed by our users starting on June 5. But, before that date some of our users were having problems accessing Read Only shared calendars, but not Managed ones.

All of our users reporting this problem are using the latest iCal 3.0.3 (1244) with ZCS 5.0.6. We're unaware of any of our users who are not experiencing this problem.

Any ideas why this is happening?