Optical connector has been partially covered by the backplate out of the box - simply disassembled the device and reassembled it correctly.
This is what my original problem has been - optical connector is unuseable this way.
instead of returning the unit to the online shop I decided to fix it myself ;)
I opened the device as it is visible in the teardown.
removed the circuit board
and reinserted it - taking care that the optical connector get's at the correct position.
10 minutes later I've finished my work and the result is perfect :)