skewed prints + XYZ calibration failed  

skewed prints + XYZ calibration failed

Hello,

I am working in a University lab, we have recently bought 2 kits of MK S3+ and assembly and printing gave no problem at all until two days ago only with one of the printers.

One printer started to print skewed objects and at first, it was only a couple of layers. So I ran calibration and it returned "XYZ calibration all right. X/Y axes are slightly skewed. Good job!" when on the first ever calibration after assembly it said "XYZ calibration is ok. X/Y axes are perpendicular. Congratulations" instead. Despite that, the skewed part of the printing started increasing, and now it's about 0.5 cm; after that it continues to print vertically, as normal (see attached pic.).

What's more, yesterday afternoon XYZ calibration started repeatedly failing in finding the last of the 4 points (rear left). 

Self-test is ok, mesh bed is ok with both 3x3 and 7x7, Z calibration is ok, and PINDA is assembled per assembly manual instructions (indeed all the printed objects were good before). I checked all the screws and the belts and they are ok. I tried dismounting and re-assemble the heated bed but it did nothing. I really can't figure out why the XY plane calibration is failing. The only thing is a high XY skew value = 0.21° but, also, I don't know how to fix that.

Can someone please help me? 

Thanks a lot!