FP2 Won't turn on or charge - See first post for troubleshooting

I decided to put my problem here, although it’s also similar to Screen blurs and lines appear and to Fairphone 2 unusable while charging.

I’v used my FP2 for one week. At first, every thing went well with charging and the screen. After 5 days, my FP2 started flicker and after some seconds, the screen turned black and I couldn’t switch it on again. After a while, I could switch it on again and everything was ok again. This happened especially while charging.
Yesterday evening it was the same, so I thought I’ll switch it on this morning, but for this time, it didn’t work. If I charge the phone, there’s only the red LED which turns to green after a while, but nothing else happens. I tried several things:

  • I removed the battery and put it on again > no change
  • There is no difference between charging with a charger or with USB to PC or to MacBook.
  • Long press on power button > the phone vibrates, but nothing happens
  • Long press on power button and volume button at the same time > the phone vibrates and there’s a sound, but nothing happens
  • I removed the screen like it is described above, and after this, the phone worked again! But after 1 oder 2 hours, there was the flickering again (now without charging) and the phone turned off. And I can’t switch it on again.

Is there anything more I can do?