1. Opened up the settings on my phone
2. Turned off NFC (I'm not sure if this is really needed.)
3. Opened the Google Cardboard App I wanted to play
4. Got an Dialogue with "Incompatible App - This Cardboard-App is not compatible with Daydream-Headsets" 5. Clicked on Cancel -> The app closed
5b. Clicked on connect device -> got a screen to scan a QR-Code
6. Scan this QR-Code:
7. Put my phone into the headset
8. It worked.