@anon9989719 , @Roms : I still can’t belive it, but it seems you were completely right!!
What happened: Since I got frustrated, I ordered the bottom module for replacement. Having this as a backup plan I was not afraid to open up the modules and have a closer look inside. But I coudn’t find anything. Everthing seemed completly ok.
But today afternoon I had the chance to use a strong illuminated magnifier and had a look at the inside of the bottom module again. And then I saw it: in the back of the USB-C port, where the contacts are soldered to the board, there was very little corrosion, that propably shorted two pins. My first attemts to clean it up failed, so I soaked the whole module in alcohol and then tried to rub the place with a toothpick. That kind of worked. The place is not clean, but the pins are not connected anymore.
Long story short: So far, about 3 hours after cleaning, everything still works fine. No charging problem, no blinking flash, no wrong notification, nothing…
Thank you again for your help and for insisting on cleaning carefully!!
Having the chance to use this huge magnifier was really good luck. Otherwise I would have never seen the corroded place…