Your boiler is having trouble controlling the temperature for one of your heating zones, likely due to a faulty sensor reading.

Mixer closes flow temperature sensor, heating circuit 2 (with mixer)

What causes this fault?

This fault occurs when the communication link between your boiler and the hot water tank is broken, meaning the boiler no longer knows how hot your water is. The most common reason is that the internal wiring of the temperature sensor has become brittle or snapped, or the sensor itself has suffered an electrical failure. Because the boiler cannot safely monitor the temperature to prevent overheating, it stops providing hot water as a safety precaution.