Hi Lucas,
It does look as though your controller is at fault.
I would suggest that there is a significant current flowing from R8 both to the microcontroller and the thermistor. If the microcontroller was functioning as expected then the current from R8 to the microcontroller would be negligible but i think in your case something has ‘cooked’ within the input circuitry on the micrcontroller and this current is now pulling down the measured voltage at the sensor terminals.
I would get onto Velleman and wait for them to reply…