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.