Google Nexus 5 — Problems with BT-connection since 8.0 Update

Dear forum,

I have been using my Nexus 5 via BT-connection in my car to listen to music while driving.
In the past everything worked great. I get in the car, turn on the radio and music starts playing automtically.
Now, I updated to Android 8 and the BT-connection is messed up. Even though the connection is established correctly (car and phone shows BT-connection is good), there is no sound. By chance, I found out, that switching the radio to the "radio station mode" and then back to "BT device", sound playing from Spotify works. But when I try playing something from Google Play Music, it does not work.

Of course, I have already tried to re-connect the car with the phone. In fact, several times. Didn´t help.
I have also tried to disable/enable the "High audio quality"-Option on the phone (under BT devices/settings).
Phone, media and contacts are also activated.

Any suggestions or similar problems?

Thanks!
Michael