Ok I see now.
You have exact duplicates of the Music files physically on both the QNAP and the Piccolo.
Correct, the libraries are identical, and the format of the music is consistent (44,1 kHz 16 bit FLAC)
If you play a file from the QNAP to the piccolo with JRemote it doesn't work. At all, ever?
Yes, from the QNAP to the Piccolo works fine, via JRiver. The the QNAP is connected to my router via LAN-cable, the Piccolo via WIFI.
The QNAP and Azur 851N are both connected via LAN-cable to my router. When I select the 'zone' of the Azur 851N networkplayer, I can see that the QNAP and Azur 851N make a connection, as I can see a song being loaded on the display of the Azur 851N. But is doesn't play, it stays on "stop" and the ID-3 tag is not loaded as it would normally (i.e. song name, artist and album cover visible in display).
When I do the same between the Piccolo and Azur 851N playback of the music works well.
In the alternative, I have also tried to link QNAP and the Azur 851N via the mConnect iPad DLNA app, and this gives the same result. I can search the database on the QNAP, but not properly link the two devices together, there is connection but no playback. Hence I think the problem can be isolated to a setting in the outgoing stream of the QNAP / MC. On the Azur 851N no settings can be changed, and the device works well with Piccolo and other sources (such as Spotify Connect).
If you tried to play that same file on the QNAP itself (say via the HDMI connection) does it play?
If I play a file on the QNAP / MC in the zone "player" it does work, via an HDMI cable to my TV over the speakers of my TV, controlled via JRemote.