Thank you for your reply.
I pressed the buttons as you suggested with the ouverture in standby and then with the unit on. I then pressed the buttons one after the other and then again together. The standby light did not blink. The panel containing the sensor is perfectly clean. And the little light in that panel when the unit is on did not blink either.
I guess that the sensor is broken or faulty. I do not want to take the unit to pieces. Are there any more suggestions please ?