Hi Matthias,
This problem most of the time means that the battery and/or screen has either been damaged or has unmatching voltage. Or another reason is that you could have damaged the proximity sensor. If the proximity sensor has been damaged. the screen may flicker or turn off. You can try unplugging and replugging the sensor, or replacing it. There are many tutorials online to help with the replacement of the sensor. I hope this finds you well.
With Regards,
-ziamaxim