Vaillant eloblock problem?

hello.
I have a 6kw water heater from vaillant and it recently shut down from an overheat protection, (F 20 displayed) which I don't know when or how occured. I flushed it with cold water through the bleed valve on top of the water heater. I made sure no air got in there. I've read on some other forums that F20 also displays when the thermoresistor fails. Does anybody know what's wrong with my dear boiler?
i'll be very pleased with any answer at all.