The rear camera was broken, probably due to a drop on a hard surface.
The repair was much more complicated than expected, but after a day of use it seems like it turned out well. So, if you're inexperienced like me.. Don't try do this repair unless you are willing to buy yourself a new iPhone.
Clearing the kitchen table after kids had gone to bed this was a different yet challenging experience. Learning something new but also understanding what is inside the gadget you're so dependent on. My only question was: Who actually makes these phones, robots or children!!? Even my slim pianofingers were a lot too clumsy to manage to place the small 1,2mm phillips-screws back in the holes at first or fifth attempt. Needless to say: It took a lot of trying and several hours to succeed my repair.