Your boiler has shut down because a safety sensor has detected that the burner area is getting too hot or is failing to register a secure connection.

Burner thermostat is open or not connected; check the thermostat for the switching point 107˚C. Check the burner cover for defects, it is possible that the flame will extend to the rear-side of the burner cover. Check the sealing between the mixing chamber and the heat exchanger and check the connection of the burner housing to the heat exchanger.