@Valentin, I'm not sure that you understand my comments. I don't believe that U1 is always installed. On those occasions where it is not present, there would be two links between the two sets of input and output pins. If these links are present on the underside of the PCB, then you have a different problem, most likely in the opto parts.
I would slip a card between the opto parts and measure the voltages at pins 4 and 6 of U1.