I had given up already but I tried the aluminum foil trick and it seems to work here!!
Those who want to try:
- cut out a small strip of alu foil (about 3 x 1 cm)
- fold it (you now have 1,5 x 1 cm)
- place it on the inside of your back cover. About halfway, on the right outerside there is a contact
- make sure it sticks out about 1 mm
- place your phone in the back cover
- check Touch Test app for the results
On my phone, this was the only thing that worked. Improvement is 90% here. Let's hope it stays this way.