If anyone else is having the same problem:
I also tried a different optical cable, a different audio receiver with optical-in etc..
The solution for me was to unplug the TV (not just power off via remote) and turn it on again.
Sheeesh... I'm glad its working now.
My audio receiver is connected via an optical cable to the SPDIF Out of the TV.
Option is set to output the audio to the external device.
When I change menu-items the "Beep"-Sound is also correctly coming through my Audio-Receiver, which means the connection is working.
I just have no idea why it seems like the current audio from the TV program / or even Apps is not being passed through.
(I'm using the inbuilt Tuner of the TV)
Thanx for any help!