Opal Triton 10.5W - Heater B switch shuts off?

Hi!
I have the Problem that my shower (Opal Triton 10.5W) does not run on the high heat ("two red flames") anymore. It still runs on cold and on medium heat ("one red flame"). Today I have opened it and looked what happens when I switch it on; For medium heat it switches on heater A (upper one) which works perfectly well. For high heat it switches both heaters on, A and B, but the switch for heater B immediately shuts off again.
I closed the switch manually and the heater heated up the water in the heater. Leaving the switch alone though, immediately switched heater B off. I asume it is an issue with the switch but I am not an electrician. Does anybody happen to have had similar problems and what are my options?

Thanks a lot in advance!