Heinz, how clean are the discs? Is there any signs of areas where the pads have stayed in contact leaving a pad shaped area visible?
If the car has been idle in the garage for a while then the small pads gap to the disc may lead to this area being affected. Many owners report brakes sticking slightly after a period of non use. This sticking can leave a rough/lightly corroded area where the pad meets the stationary disc, and this then shows up as a shudder, especially at any sort of speed. I had this many years ago on a car after a period of non use. New discs and pads needed!
