Unable to initialise PositionalDeviceTracker and SmartTerrain

Hi, I'm trying to integrate Vuforia version 7 into our own iOS application. Object tracking already works for us. Now I want to utilise Ground-Plane, but initialising PositionalDeviceTracker and SmartTerrain just returns null.

PositionalDeviceTracker and SmartTerrain are not initialised already at that point, the device supports these trackers (Vuforia GroundPlane sample app works without any problems). Vuforia and camera are correctly initialised before. I tried do verify that behaviour by disabling video background initialisation, camera and AR initialisation on that Vuforia GroundPlane sample project. Surprisingly initialising PositionalDeviceTracker still works with that modified sample project ( does not return null). Does PositionalDeviceTracker needs device capabilities maybe we are already initialising or utilising with our iOS application? The iOS Device we are using is an iPad mini 2 with iOS 11.2.5 - on that Vuforia GroundPlane sample App works fine. Initialisation code looks like the one from Vuforia GroundPlane sample.

// initialise vuforia
// initialise camera

Vuforia::Type trackerType = Vuforia::PositionalDeviceTracker::getClassType();
deviceTracker = static_cast<Vuforia::PositionalDeviceTracker*>(trackerManager.initTracker( trackerType )); if ( deviceTracker == nullptr ) { return; }

Thanks!