Your boiler has lost connection with its main internal temperature sensor, meaning it cannot safely tell how hot the water is and has shut down as a precaution.

Flow–NTC: NTC broken, NTC cable broken, Defective connection at NTC, Defective connection at electronics | Cause: Flow–NTC cable defective/broken NTC faulty

What causes this fault?

This fault occurs when the boiler's internal computer can no longer detect a signal from the sensor that monitors water temperature. The most common reason is that the sensor itself has simply worn out over time, though it can also be caused by a loose connection or damaged wiring inside the unit.