extruder temp is reported wrong. phatus rapido uhf

I just did a 38h long print and now after cooldown to 23C my printer says that extruder temp is 51c. So i took the toolhead apart and hot-end is cool to touch. If i take it to my hand and let it warm up, reported temp increases as expected. So i took out my multimeter and check the residence: 102kOhm at 23C. If i look at the datasheet, it checks out. What can i do?