Here is the report I promised: Not sure how to check if all contacts align well though since I can’t look into the phone when locking the screen module with the rest 
but I followed Paula’s idea of cleaning the contacts.
This helped a lot. Both Ethanol and Isopropanol do a good job here. After cleaning the contacts and putting all parts together again, I works well without flickering for about two or three weeks, then the flicker comes back. Instead of flickering, sometimes the phone produces graphic noise that modifies the screen content over a period of a few seconds so that it gets darker and darker, or lighter/coloured, or repeats part of the screen content over and over again. At other times or after this, the screen flickers heavily between full white and its supposed content. This does not only affect the graphic content, but also the touch functionality (doesn’t work) and the on/off button (doesn’t work) and the back(!) camera (graphic artefacts). So, if you experience the screen flicker and sometimes your phone does not display anything after pressing “on”, it might well be in flicker mode but with the display switched off.
I tried the cleaning three times meanwhile - I am too lazy to clean directly when the first flicker appears, but it gets worse over time and when it is at a level that renders the phone more or less unusable, I clean it again and it will work for some time.
Of course I would expect the flicker to disappear permanently. I think about filing a support request and asking for (display module?) replacement, but wonder if this is state of the art and a high percentage of users has this issue. Any suggestions?