No edit possible for CalDAV entries created on the Zimbra web front end

[mfehr]

I have the following situation: We have 2 iPhones, every one a private Zimbra account as well as we have a shared Zimbra account as well. We both have admin access to the shared account for our shared calendar entries. The idea is that everyone can manage calendar entries either for one self or for us together. From the iPhone, we have 1 CalDAV subscription to our personal Zimbra account. This gives access to all subscriptions on ZCS down to the iPhone.

Creating a calendar entry from my calendar on the shared calendar (using the ZCS web access) also replicates the calendar entry down to the phones. However, these entries cannot be edited. They show up as an invitation from the shared account. No changes are possible on the iPhone.

Creating a calendar entry on my personal calendar the same way works as expected. Creating a calendar entry from the iPhone on this shared CalDAV calendar works as expected. I can edit/delete the entry on the other iPhone and of course I can also edit/delete it on the ZCS web front end. - So the issue is only with shared calendars.

On the server, I run ZCS 5.0.18 on Ubuntu 8.04. Any hints how this can be solved or is it already solved in v6?