thank you all for your advice.
more or less I actually had the same information about reset. But it does not work! So I thought there is a special trick I didin't yet try to hold the reset button for more than a minute, but definitely longer than 30 seconds. And in any case, it's not easy to safely hold the button as long.
As nothing seems to help, I obviously have to live with this defect. But I am still wondering wether nobody else has encountered the same problem.