Print bed thermistor constant 170°C reading ... worked before ;(

Hello,

I have a problem with my print bed thermistor, or better with the temperature display in Marlin.
Everything worked in the beginning
HotEnd Temp was displayed, bed temp was displayed I could heat up both and did the PID tuning. Then days later I got a MaxTempError Bed and the display shows constat 170°C

I check the resistance on both thermistors (HotEnd and Bed) both are aroun104kOhms

If I switch the plug and the heatbed thermistor goes into T0, the bed temperature is display as hotend on the LCD and is "heating" up if i press my thumb on it.
So I guess the thermistors are not shorted and working fine.

The temperature for the bed stays the same even if there is no thermistor pluged in at all
Hotend drops to 0 but the bed stays at 170

I am completely lost at the moment because it worked in the beginning and now I have this issue

config settings:
#define TEMP_SENSOR_0 5
#define TEMP_SENSOR_1 0
#define TEMP_SENSOR_2 0
#define TEMP_SENSOR_BED 1

E3D hotend and a reprapworld thermistor B57550G104J

Any hint or tip or procedure to fix this is greatly appreciated

Cheers
Axel