Recently, the calibration process of my Mini becomes inaccurate. The nozzle is always a little bit lower than expected, and this can cause terrible results in the first 5 to 10 layers:
As you can see, the skirt is abnormally wide, because the nozzle is squeezing the filament onto the printbed. Also, the nozzle is stirring in the printed part because it is ~1mm lower than expected. (This has nothing to do with the printing tape I added. Its thickness is negligible compared with the height error of the finished part.)
The situation could become better after it starts printing the internal supports with cavities, because the overflowed material has somewhere to go.
In the end, my printings end up with terrible surface finish on the bottom, and an unacceptably large error in height.
How can I solve this? The calibration process is automatic and I can do nothing with it. Where could things be wrong?