Mopdiy server does not play music

I just installed Mopidy on my Ubuntu 16.04.2 LTS headless server (an old desktop PC) and I can’t play any songs.

Pulseaudio is working an the music files are valid files. I’ve tried to get songs to play using Chimney, MPDroid, MPDi and other clients. They all discover the server automatically and show the full library (which at this moment is only some 30 items for testing purposes). No songs get added to the playlist and clicking play doesn’t give any response. The log file shows only [INFO] items, nothing weird.

What am I missing?

● mopidy.service - Mopidy music server
   Loaded: loaded (/lib/systemd/system/mopidy.service; enabled; vendor preset: enabled)
   Active: active (running) since Sun 2017-07-02 18:37:45 CEST; 1min 28s ago
  Process: 1589 ExecStartPre=/bin/chown mopidy:audio /var/cache/mopidy (code=exited, status=0/SUCCESS)
  Process: 1569 ExecStartPre=/bin/mkdir -p /var/cache/mopidy (code=exited, status=0/SUCCESS)
 Main PID: 1593 (mopidy)
    Tasks: 10
   Memory: 33.4M
      CPU: 406ms
   CGroup: /system.slice/mopidy.service
           └─1593 /usr/bin/python /usr/bin/mopidy --config /usr/share/mopidy/conf.d:/etc/mopidy/mopidy.conf

Jul 02 18:37:46 helicon mopidy[1593]: INFO     Starting Mopidy mixer: SoftwareMixer
Jul 02 18:37:46 helicon mopidy[1593]: INFO     Mixer volume set to 100
Jul 02 18:37:46 helicon mopidy[1593]: INFO     Starting Mopidy audio
Jul 02 18:37:46 helicon mopidy[1593]: INFO     Starting Mopidy backends: StreamBackend, M3UBackend, FileBackend, LocalBackend
Jul 02 18:37:46 helicon mopidy[1593]: INFO     Loaded 26 local tracks using json
Jul 02 18:37:46 helicon mopidy[1593]: INFO     Starting Mopidy core
Jul 02 18:37:47 helicon mopidy[1593]: INFO     Starting Mopidy frontends: MpdFrontend
Jul 02 18:37:47 helicon mopidy[1593]: INFO     MPD server running at [::ffff:0.0.0.0]:6600
Jul 02 18:37:47 helicon mopidy[1593]: INFO     Starting GLib mainloop
Jul 02 18:37:47 helicon mopidy[1593]: INFO     Audio output set to "autoaudiosink"