It is possible that some dust got between the camera and the display. You can try taking it apart and cleaning it with compressed air. Also check for any loose connectors.
So, after opening my phone and camera compartment, I found out what it was. Somehow a sticker (yes you read that right!) had shifted before the eye of the camera. I have never opened the camera compartment before so I have really no idea how it got there. It was quite sticky and it was partly (with the long end of the sticker) tightened between the backside and the screen of my phone.
The sticker had the same size as the front camera, as you can see on the added pictures.
One more addition from my side. I guess the sticker came with the new screen that I put on my phone. However, I got some help with the repelacement, and we didn’t see it when we replaced the screen. A bit weird huh?