I have an AEG competence 3200F-W cooker (KD 611.251 855-076/841467). The largest hot plate on the top will only work on highest temperature even when adjusting the setting on the knob.
I think it is a faulty regulator switch, part No. 661052420. To confirm this you could inspect the switch for burnt contacts etc. If it looks OK you could try swapping it with one of the switches for the other rings to check that the symptom is transferred to the other ring. As always, power off before opening up anything.