Your boiler has failed to heat the water to a high enough temperature to complete its automatic hygiene cycle for killing bacteria.

DIY fix possible

Check if the hot water temperature is set too low on the boiler control panelEnsure there has not been a prolonged period of high hot water demand during the cyclePress the reset button to allow the boiler to re-attempt the hygiene cycleMonitor for a recurrence; if the code returns, contact a professional