Good News!
Today I received an email from Dual which simply fixed the problem with the external bluetooth gps receiver!
Step-by-step instruction:
1. Turn bluetooth-device off
2. On iOS device go to Settings->Bluetooth and find your external gps device, tap on the blue arrow, than tap on “Forget this device”
3. Close all applications
4. Hold power-off-button an swipe to shut down your iOS device
5. Turn it on again
6. Turn on your external Bluetooth GPS-receiver
7. Go to Settings->Bluetooth find your gps-receiver an tap on it to pair
8. Here we go! GPS-receiving should work again!
Thanks you guys at Dual!