Live Player lost connection on start

I'm using VST 15.5.4 Enterprise and enabled the Live Player in VST.
I installed the Xamarin Live App on my Android phone (1.4.134).
The connection test went well - also the pairing. My device is listed in the left part of the player's pairing dialog.

But if I start a simple Xamarin.Forms App via F5 and the target "Live Player" - the pairing dialog appears and after repairing the device, the program stops immediately and the android app says "Connection Lost".

What could be wrong? Port 8090 is open for both the PC and the android device (as the connection test shows) and the pairing is shown as established in the XamarinLive-App on the phone.