Your boiler has stopped working because a sensor that monitors the temperature of the water returning to the unit has detected a short circuit.

Short circuit - return-NTC (< 130 °C) | NTC- plug shorted to casing NTC defective

What causes this fault?

This fault occurs when the electrical signal from the sensor monitoring the return water temperature is interrupted by a short circuit. The most common reason is that the sensor's internal components have failed, or the wiring and plugs have become damaged and are touching the boiler's metal casing. Because the control board can no longer accurately read the temperature, it shuts down the boiler as a safety precaution.