I apologize, please ignore my last 3 posts!
The real reason was a tottering contact with the plug on the flat cable coming from the touchscreen and plugged in into a jack on the main PCB.
So I opened the device and fixed it by bending the cover plate of the plug in a way that it better fixates the plug.
now it’s working better than ever before. Let’s see how long.
edit: 22 days later , still working fine !