In the selection under "Available devices" in my Spotify app, only the zone can be selected, not the individual devices. What must be set so that I can call an individual room?

First, deactivate the Multiroom mode in the devil Raumfeld app under the menu item Settings/Music services/Spotify by setting the switch to "OFF". Afterwards, each device can be called up individually via the Spotify app.

Please also note that the Spotify Connect feature (except for the Cinebar Lux) requires a paid Spotify Premium account.