Thank you for your help, gr5!
"retract on layer change" was turned off already. I tried your suggestion varying the feedrate while printing and it turns out the ripples dissappear when tunring it down to 25%. At 50% they still appear. While this is good news, it also dramatically increases my printing times.
With my Speeds set around 25 mm/s, printing time was 40 Minutes for 5g of filament. Turned down to 25% its 2 h 30 minutes. Since I will have to print around 500g of filament to finish my project I am wondering if there is something else I could try.