I finally found some website that allowed me to configure bluez to work both as alsa and as pulseaudio device. My Bluetooth audio device is now working. The main hint comes from this post:

I didn't check everything by now, but the most important hint seems to be #2, that told me to add

Enable=Source,Sink,Headset,Gateway,Control,Socket,Media

to /etc/bluetooth/audio.conf instead of the disable option.

Additionally I put the following contents into /etc/asound.conf: