Thermistor has cracked and gotten moist. Replace it, pretty common on microwaves for them to crack. Probably best to buy from Matsushita agents though, and if they do not want to then look for one in Germany from one of the white goods spares suppliers.
I know how you feel, got a lovely Sharp convection microwave in the garage, needs a new keypad, because it is worn out. But dealing with the local Sharp agents is not nice, so when i get around to it ( i really do not need another microwave, the cheapie works fine for me with 2 knobs for use, no fancy electronics and beeping that uses more standby power than the oven ever will) i will order from Ebay and put it in, or rip the panel out and go manual control.