After studing the signal flow from Input C to actual receiver chip it came obvious that it's all but optimal signal path and badly needs redesign.
In original form the digital input signal travels from Input C XLR connector through relay contacts and then along the PCB to grey ribbon cable and then again on PCB. Alltogehter this is some 20cm of not optimal signal transmission.
I decided to disable the input C entirely as I never see myself needing it. Aslo as my setup is using all the time only the digital in of the DCX I decided to run the proper coaxial cable right to the DSP PCB.