Update: if i press the button "Home" it will correctly posizionate the nozzle above the printing bed, with a gap wide enough to insert a sheet of paper between the nozzle and the surface. But, and here's the strange thing, if i move the printing bed the z axis motor will rotate causing the nozzle to get too close to the surface, scratching it.
I recently bought my first 3D printer, a Monoprice MP10 Mini. Unfortunately not a nice choice so far.
I calibrated the printer 3 times and after every print the Z axis seems to be completely off.
As a consequence, it scratched the printing bed many times. On top of that it seems I can't get a decent print anymore, I don't know how I can describe this problem, so just look at the pictures: