Hello everyone!
Thanks for all the recommendations. I am almost a bit ashamed about the solution of the problem we found: Horrible "on the fly" grounding caused that issue!
Although everything was connected we missed a proper ground connection from the external module to our pcb with the adau . When spdif was connected the floating ground was tied a bit closer (although trough spdif isolation basically still floating) what helped enough to get a clock sync.
Now everything is in a nice case with proper ground and whooops....works ;-)
Well at least almost; but there I just open a new topic ;-)
Sorry wasting time due to such a unnecessary mistake!
Thanks everyone!
Ron