Replaced the Temperature Sensor related to the defrost cycle. Most of the time is spent steaming off the ice buildup. Hardest: to take off the back panel inside the fridge cabinet without breaking the foam insulation. The ice buildup gets the foam stuck against the plastic. Must be very careful to slowly separate the insulation from the ice.
The repair in itself was successful. However, 5 days after, the internal temperature is about 3.5 degrees Celsius higher than the target temperature. The fridge panel says the temperature is 1 degree celsius, but the internal temperature is between 4 and 5.5 most of the time. Next repair: changing the temperature sensor inside the cabinet.