Camera doesn't work. It's not the camera.

Hello together,

I bought an used iPhone 4. After some weeks the phone was dead, because the batterie was broken. So I changed it.

After this replacement, the phoned work good. One evening, I tried to make some photos with the back camera. The first pictures were no problem but then the screen was black. I tapped around on the screen but nothing. I restarted the phone, nothing, so I recovered the phone and nothing again.

As I said, the screen is black. But it's not black like the lense is blocking. If I knock on the phone, I see much purple stripes on the screen but not the motif. The light is also not working, only somtimes. I can change the HDR mode and can use the front camera.

The last thing I tried was, to change the camera&light modul. But the same failures.

I would say the connector port on the logicboard is faulty but it looks good. All pins are on the right place und there is no dust.

Any ideas, what I can do?