Old thread but I had a similar problem with a Remeha Avanta Plus 39c - which is now fixed.
With the casing off, the flow sensor led was permanently on (red light) indicating a constant demand for hot water. Pulled off the sensor, light went off. Tested this with a magnet, light back on meaning this was working okay.
The problem was the flow switch regulator. This component has a spring and a magnet inside which in my case had bunged up and stopped working.
Replaced the part with service kit 720532601 (burner cover has to come off so new seals, bolts, washers and clips). All good and back to normal.
FYI: If the boiler was set to CH mode only, the boiler worked fine. If on CH/DHW or DHW only, then the problem of the fan running on and the constant light remained. As such this indicated that the diverter was not the problem but the something to do with hot water line. The giveaway was the red light always on!