After around 90 minutes shooting M 34, the Pleiades came in sight (M 34 slowly disappeared behind the roof anyway). So I decided to switch to the Pleiades at the same settings. This way I can use the same flats and darks for both targets.
Lesson learned: take flats directly after every target! Dust or other particles can move during a slew to the next target! I had to crop my M 34 image. There was a spot in the left upper corner which wasn’t removed well with the flats.