i replaced some cables going from the 9-pin D-SUB connection block connecting the VCO with the DAQ by bi-colored, twisted ones.
Someone used yellow cables for everything when he set this up. Now the ADC and DAC channels are bi-colored (black-LO, yellow-HI).

I also removed one connection from ground to the rack frame, so protective earth. I don't see why we need that as we have one at the power supply already. There ground=earth.