RE: Mesh bed leveling failed.
Also take a look at this
https://help.prusa3d.com/en/article/xz-axis-skew-correction-mini
Original Prusa Mini + Smooth PEI
Prusa Slicer 2.6.0
RE: Mesh bed leveling failed.
I had the same problem with my brand new Mini. It gave a mesh bed failure after only three points. Loosing the three screws holding the z-axis didn't resolve it. What worked was loosing the four screws (2 front + 2 back) on the right hand side frame of the base. I then applied a sight torque until the left hand side of the print bed was the same level (relative to the PINDA sensor) as the right. I then tightened them all. This resolved it to me (no paper needed). I believe this should be part of the construction of the "pre-assembled" printer. I have built three others and had no issues like this.