Thanks for the advice. I did try printing a version scaled x2 in the z-axis and the print came out perfect. I guess this means it is not a problem with the z-screw ?
Looking at the printing process closely, the deformity seen in the pictures at the base of the thinner cylinder seems to be caused by material already deposited sticking to the nozzle as it passes again. But in the scaled print there was no such effect...
Any ideas ?