Hi guy.I don't understand German, but try measuring post voltage converter voltages, possibly the car battery booster is out of service. Depending on power and model, it must be near 35 or 40VDC.
It is strange that you have the same trouble in the 4 channels. If supply rails are ok, then you can check all common circuitry. In my cell I don't see it so clear as to say what. Images are very minute here. This evening I shall use my PC.