And the high tech advanced tool used for the repair:
You see correctly it is a cosmetic cotton pad (a dirty one).
The unit has touch screen. This called "soft keys". Because of the age of the unit it is not a capacitive or resistive touch screen but an optical grid created with infrared LEDs and phototransistors.
- When I first seen the error I think that one of the LEDs may failed (2 sec)
- Then I read the manual what the error code is about (5 min)
- When it proved that my theory is close to the possible cause, first cleared the frame of the display (30 sec)