Duet does not play the music

Since 2 years I am using the Squeezebox Duet with pleasure.
But now the music is not played anymore.
The status display of the Duet is: white bright (playing music).
On the Remote Control the played music track is running.
The display on the digital pre-amp shows: TOSLINK no lock.
When I connect the SAT decoder to the digital pre-amp the display shows: TOSLINK, and playing the broadcasted program.

My configuration:
Router: Sitecom WL-614 300N
NAS: QNAP TS-119P+ wired connected to the Router
Squeezebox Duet optical connected to the Digital Preamplifier.