Neither driver nor passenger side front windows lower when pressing the buttons on the control panel. The only sound coming from inside the doors is a relay click. No sound from the motor, no rattling sound from the regulator mechanism. The rear windows work fine. What may cause this behavior? May the control unit that is attached to the window motor (called "türsteuergerät" in German) have something to do with it?

There are no relays in the doors but the click you hear is the motor trying to move the glass but held back by the torque limit control (also know known as the finger pinch limit when closing).

 

You obviously have something in the mechanism that is putting too much load on the motor. This happens in the winter time here when ice sticks the glass to the frame. Check your battery as a weak battery will give a similar result.

 

There is an override described in the handbook of your car. Press the up or down button as appropriate for 5+ seconds then briefly release and immediately try again as normal.

 

If this does not work then there is a serious blockage to movement that must be investigated inside the door.

 

One last thing you can try is have someone put the flats of their hands each side of the window glass [with the door open] and try to slide the glass down while you operate the switch.

 

Thanks for your respons and pointing out the reason for the click sound. I should have known. The click naturally comes when I lift the switch to close windows that are already closed. No click sound and nothing else happens when I try to open the windows by pressing the switch. It's also not possible to slide the window down using flat hands against the glass while operating the switch. I think that a blockage, a broken regulator wire or a motor failure at the same time on both front windows is not very probable. Other reasons for the problem may be with the cables or the electronics. I can get hold of a used motor/control unit for about 40 Euros. Does it make sense to try a replacement of that part? And is it possible to do that without removing the pop riveted wall and without screwing up the regulator mechanism behind the wall?