Your boiler is receiving a instruction to heat water to a specific temperature from an external thermostat or smart heating control.

Flow target temperature from external analogue regulator terminal

What causes this fault?

This is not a mechanical failure, but rather a status message indicating that an external control, such as a smart thermostat, is telling the boiler exactly what temperature the water needs to be. It occurs when your heating controls take over the management of the boiler's output to improve efficiency. The boiler is simply acknowledging that it is following these external instructions instead of its own internal settings.