Your boiler hasn't been able to heat the water hot enough to safely kill off any potential bacteria in the system.

DIY fix possible

Check that your hot water timer is set to be on for at least one hour to allow it to reach temperatureEnsure your hot water temperature dial or cylinder thermostat is not set too low (should be at least 60°C)Check if you have used a large amount of hot water just before the scheduled anti-bacteria cycle