First of all I was trying to test the ShowCalloutAt on the Mobile Device with a simple Text Message:
MapView.ShowCalloutAt(p, new CalloutDefinition("Hallo"));
This works fine on the Emulator but also does not work on the Mobile Device.
When moving around the map i was able to see, that it is a problem with the positioning of the popup. The anchor point of the popup is always the left upper corner of the map and not where the the feature is located.
The MapPoint (p) has a correct coordinate and spatial reference.
Any Idea what could cause the problem with the anchor point?