AEG Arctis 75100 freezer problem

After carefully clearing an ice build-up from the inside bottom of my freestanding AEG Arctis 75100 freezer, the alarm went off a few minutes later (understandable in this hot weather). However, the temperature now only goes go down to -16 and the alarm sounds and whenever I open and close the door, even for the shortest periods. The alarm also seems to go off at random times during the day and night.

I read on this forum that a similar sort of problem occurred on the upright version of this model, but is the problem and suggested solution (ie check the defrost drain heater) still applicable to this model?