iPhone does not sync with CalDAV

Hi,

I've a iPhone 4S and trying to sync with a shared CalDAV calendar.

I'm accessing this calendar additionally with Mozilla Sunbird and another iPhone (4).

The calendar rhad already entries created by Sunbird before.

I don't see these entries in my iPhone. But when I create a new event through the iPhone it appears quickly in Sunbird. A new entry created by Sunbird does not appear in iPhone.

A new entry in iPhone 4 does not appear in 4S. And vice versa.

So it looks like the iPhones update their own events but never read from the network.

Any clue what might be wrong here? I've tried to set the "Read" interval to 30 minutes but no change...

Thanks!

Christian