Yeah, thank you Svenne !
I followed your instructions but it did not work. So I retried with a step more and it worked. So for those who have this problem in the future, the following procedure:
- Turn of the device
- While keeping the pad button and rotary knob 1 pressed, turn on the device
- Wait for the LEDs stops flashing and 3 LEDs light up around the rotary knobs
- Still holding the pad button and rotary knob 1 pressed, press the rotary knob 2 and wait the the LEDs stop flashing.
After that, my MR816X works great again !