Just for the records... HP support came to see my printer. And the constant 52:10 error message (Cleaning needed. Clean Drop Detector...) was caused by ink on the Encoder. I also had the Color Sensor 58:10 error code, also probably cause by ink that was on the encoder.
So... if your are planning to move your printer as I did (2 floors down, 2 floors up, by hand, no elevator) take care that any ink gets the Encoder (better yet... dissamble the right side of the printer to check if there is any ink in the encoder)
After the cleaning, my printer was in the road again, reliable as it was before.