Your boiler has failed to heat the stored water to a high enough temperature to kill off bacteria, usually because the hot water hasn't been used for a while or the heating cycle timed out.

DIY fix possible

Check that the hot water temperature is set high enough on the boiler control panel (typically 60°C or above).Ensure there is no current high demand for hot water elsewhere in the home during the disinfection cycle.Restart the boiler to allow it to attempt the anti-legionella heating cycle again.