Your boiler is providing an advisory notification about how much power it is using to heat your hot water, rather than indicating a mechanical breakdown.

What causes this fault?

This notification occurs when the boiler is informing you that the maximum power output dedicated to heating your hot water has been manually adjusted or restricted. It is not a mechanical failure, but rather a configuration setting that tells the system how hard the burner should work when refilling your hot water cylinder. The most common reason for seeing this is following a recent service or setup change where the engineer has optimized the unit for energy efficiency.