Your boiler has detected that there is too much water pressure in the system, which can cause internal leaks or damage if not reduced.

DIY fix possible

Locate a radiator bleed valve and use a key to release water into a cloth or containerMonitor the pressure gauge on the boiler until it drops between 1.0 and 1.5 barEnsure the filling loop valves are fully closed and not letting in more waterPress the reset button once the pressure has been lowered