Thank you for your advice!
Updating didn´t change anything, but then I turned the "protect channels" on, which was not turned on, and set "positive list". Since that moment I am asked again for my pin when trying watching TV channels or the media player.
Maybe this setting was lost when I flashed the new Image a week ago...
Interesting, that watching sth. with media player is protected only when the channels are protected. So I asume
config.ParentalControl.configured=true
config.ParentalControl.setuppinactive=true
config.ParentalControl.setuppin=...
are - as long as they are the only settings - just preventing kids from setting up the control for the adult
BTW, I removed the kiddyblock plugin because of its unknown original pin which prevents any configuration...