Siemens EQ.6 - NTC Surface Sensor Error

Hello
,my EQ.6 plus s700 (TE657503DE) turns off today (After a cleaning of the brew group) again and again. Sometimes directly after switching on, sometimes in the preparation process. It is not possible to prepare a full cup of coffee.
The error log says "NTC Surface Sensor Error". Nothing else. Is this the flow heater?
In the sensor menu there are values for input and output. Both values change in a plausible range (50 and higher). If I start a water heating in the component test, the temperature also rises to >100.
Somehow this contradicts the message "Sensor Error". I would expect that there is no value or a totally implausible value. Before I order now on suspicion a thermoblock and Tasusche the question if someone knows the error pattern?

Thanks a lot

NTC are the temperature sensors on the instantaneous water heater, one at the bottom and one at the top. At room temperature, both should have a resistance of between 50 and 60kOhm and should be as close to each other as possible.
But it can also be the circuit board.