Black screen after screen replacement

Hello!

I'm having strange problem in my iPhone 6 after screen replacement. I replaced screen normally but after connecting cables to motherboard it shows only black screen. But if i leave proximity sensor flex cable unconnected phone starts normally. Also if i unscrew the whole flex cable away from front panel and connect it while screen is also connected there is no problem. Then screw it back to front panel and black screen again.

Tested with my old cracked screen. There is no this problem. It works as it should work. This is my 4th screen replacement to this phone and i've never seen same problem?!