[Marcus]

Hi Guys,

first of all thank you very mutch for your valuable inputs! Indeed the driver side window was not operating because of a broken white plastic clamp supposed to keep the cable in position (like Earthworm pointed out). The passenger side was not operating because of the faulty switch. I replaced the switch (like svxcess suggested) and the clamp and was able to operate all windows again.
Unfortunately the driver side window is still giving me trouble, it operated only with squeaking noise up and down, now it goes only about half an inch down and gets stuck in this position. So I guess the broken plastic clamp is not the route cause, but the consequence of some kind of irregular friction. Given this I have to remove the door panel again...