iPhone (using iCal) is able to send Calender entries to Zimbra, but does not sync entries created via Webinterface.

The iPhone is set to check the calendar every 15 minutes and does so. Looking through the logs and traffic everything seems to be in order.
The only thing that seems a bit weird is this part of the response:

<C:calendar xmlns:C="urn:ietf:params:xml:ns:caldav"/>
<Y:default-calendar xmlns:Y="http://yahoo.com/ns/"/>
Is this default-calendar correct?
Is this the reason the iPhone doesnt sync the entries?