I tried different temperatures, speeds and layer thicknesses. No luck. I noticed that the messiness starts at a spot where the z distance varies and I think that might have to do with a "flat tire" on one of my Rostock Max V2 cheapskates. I think I tightened it too much and one of the bearing sleeves got "sticky". I loosened the tension and the stickiness went away. I am now trying to see, if the prints in that area will get smoother.