after repair, display is not working anymore

Yesterday, I changed the display/digitizer unit because the original screen showed a grey vertical bar on the left display side. I worked accoring ifixit repair guide.

After reassembling, the display stays black, only the background lighting comes up. Beside this defect, the phone seems to work properly - it is recognized by itunes and i can flash/synchronize.

What I tried:

-I tried to reset (home+power) for several times

-I tried to reinstall firmware (6.2.1)

-I disconnected all non relevant connectors for testing basic function (i only left power button, display and battery)

-I reconnected the old display unit

But nothing helped.

Did I brick my iphone or does anyone have a hint, how to get the display working again? Many thanks in advance!