Update: after some non-successful attempts I decided to apt-get update/upgrade the fresh install – again without luck in terms of Spotify Connect. Then I deactivated the Spotify Connect plugin via the webinterface and installed https://github.com/dtcooper/raspotify – now the device is visible via the Spotify Android app as raspotify (max2play) and music playback works fine.
Don’t know how this install will influence max2play. However, Spotify Connect is the only reason so far to use max2play.
Awaiting advice to replace this workaround.