IPhone 4 strange camera problem

Hi guys

I am facing a strange problem wit the rear camera of an iPhone 4: the screen stays black when opening the camera app, there is no IRIS visible, camera functions remain available and active (autofocus frame is visible, format switching is possible, HDR On/Off, Flash on/off).

The following troubleshooting options have already been checked:

Software side:
– full RESET of the settings/camera
– privacy settings (made sure the camera is not blocked by an app)

Hardware side:
– 3 different cameras have been checked, all of them are original apple made and not OEM replacement spares
– all cameras have been checked in a working Iphone 4 and confirmed to work perfectly
– when built into the defective Iphone, screen remains dark in two cases and with the third cmaera you see a streaky and blurry picture that has nothing to do with the crisp and perfect picture of a working camera.

After testing 3 cameras, which are confirmed to work properly, I am sure, that the fault cannot be related to the camera hardware.

– the small SMD component near the display connector is OK and not damaged (see certain YouTube videos which refer to this component).

– connectors have been checked and no damage has been found, neither on the camera main-PCB nor display connectors.

Maybe someone of you specialists here have an idea and can help me to narrow down the problem a little bit more ?

Thanks in advance for your help !