Layers print at half height

A strange problem has suddenly appeard: our Prusa with Marlin firmware on SAV.Mkl suddenly prints every layer with half the height.

It happens

-with different files
-with different slicers
-printing over SD
-printing over USB.

What could I try next to find out what causes this? My son was trying to print something with 0.1 layer height instead of the 0.2 or 0.25 we usually print with - problem started after that, but not sure it it is related.

Further test I did:

Sending G1 Z10 just lifts the Z-axis 5mm.

Should I calibrate it? But how could it stay de-calibrated if everything was working fine before?

I had a similar issue when I first built a smartrap mini, but my issue had to do with the pot not set correctly on the stepper DRV8825 boards. You could also check your firmware steps to make sure that is still setup correctly at least.