first of all thank you for your answer.
1. the cable what Iam using is an coaxial cable which should work for a test. The surge impedance isn't optimal (I think this is around 50ohm and the optimum is around 75ohm?!) but it should work for a test. my proof: The signalflow from the interface to the kemper is working. Please keep in mind this is my first contact with spdif, so I have no experience with this standard and didn't knew about the so specific "SPDIF cable".
2. I don't have to increase the SPDIF volume because 0.0dB is the maximum. There is just the possiblity to damb it.
3. I have read the manual about my issue und could not find the solution. I also could not find the "SPDIF out link" feature.
Also have read a lot of threads, seen clips but no advice about a specific cable.
All I can say is that that was not my best post on this forum. Will not delete it but probably I should. Too many mistakes and misunderstood bits.
In my defense it was late and I probably wasn't in the best mood. Should have gone to sleep and try to help you in the morning.