Back to the original topic. I finally found some time to play with the printer again and tried your suggestions.
I have forced the steps per mm to 64 and did a test print. It did not fix the problem but the extruder motor was louder.
I have printed 2 hollow test-cubes next to each other. There were no signs of under extrusion but the retraction cylinders looked bad.
Finally I have decided to just use the extra prime amount for the time being. It gives me the good results inspite of my reluctance to use it.
Thank you for your input. I learned a lot about the finer details of extruders.