Your boiler is confused because its internal thermometers are giving conflicting readings and don't agree on how hot the water actually is.

Fault: Flow/return temperature sensor Values not consistent, difference between flow and return <-6K Flow and return temperature sensors signalling implausible values: Flow and return temperature sensors have been inverted, flow and return temperature sensors have not been correctly installed

What causes this fault?

This fault occurs when the boiler's internal sensors provide temperature readings that are too far apart for the system to trust, suggesting the water is heating up in a way that doesn't make sense. The most common reason is that one of the sensors has developed a chemical or Electrical fault, but it can also happen if the sensors are incorrectly installed or have become loose from the internal pipework.