Miele G1022 giving brief pulses of power to water heater

My Miele G1022 stops washing with three Wash / Rinse, Drying and End LEDs flashing at 1 Hz.

• Heater element is 25 Ω. That looks OK.
• Thermistor is 13 kΩ at room temperature and 2 kΩ in a cup of boiled water. That looks OK.
• The Schrack 0419 11 0101 02 relay shows scorching on the inside of the case but is working. I've hooked up a mains LED lamp in parallel with the element and it's switching OK but only in short pulses. I've also tested operation with a small Siemens contactor with the same result.

When I test the machine on a 40°C cycle I get this:
m:ss (time)
0:00 Start drain.
0:28 Water in.
2:00 Start wash.
2:07 0.5 s flash on my heater monitoring lamp.
2:12 Stop.
2:22 Start wash.
This start wash sequence repeats 13 times and the error signal turns on at 6:30.

Accessing the diagnostics as I have documented on another thread, How to display the Miele G 1022 fault code, I am getting five flashes which (I think) indicates a U5 fault. The G1000 / G2000 Technical Information (which is the closest I could find) says:


I'm going to have a look at the flow meter but has anyone got a better idea?