Ok, I found the bug. I don´t know why that is so, but never mind. I changed the wire orientation of the thermistor and that was it. Is there any diode in the thermistor or why the orientation matters? I thought that a thermistor is a restistor with a almost linear restistance dependency to the temperature.
I thought so too - it shouldn't matter which wire goes to which side of the thermistor. Maybe you have a bad connection somewhere which coincidentally got disturbed when you were swapping wires? I can't see anything obviously wrong with your configuration.