Thanks but I was just being dumb. The selected playlist was empty, so it was impossible to play any music as expected.
I confirm what you said about the device list but only if I edit a previously saved task with the “offline device”, created when it was online. Creating a new one doesn’t show the non-connected devices as expected.
Anyway, thanks for the fast reply and many many thanks for creating the best way to wake up, using a Synology!