Replaced display is dimming to dark

I replaced my display with an Ifixit replacement part and I did put in a new battery.

Everything worked fine (it is the 3rd time on this phone)

After a while I realized, that the display was showing some flickering which can barely be noticed.

I turned the brightness up and down to see if it would went away and then it got so dark that I couldn't turn it bright again.

After I finally got it back lighted I turned of the brightness adaption and rebooted several times.

When closing the phone everything seemed connected well.

There has been a screw - right above the Wifi-Cable connector which seemed loos and I tightened it.

Don't know what to check for.

The display goes dark, as soon as I turn down the brightness to 25-30%