Black screen after volume flex replacement

Hi guys,

I have been repairing iPod Touch 4g for some time now but I ran into something that I apparently cannot fix.

I had some iPods where I changed the volumeflex cable but after that I only got a black screen.

The ipod is turning on as I can tell this by the sound and iTunes but no reset, update or whatsoever helped.

I found the post that you should desolder the + on the battery but that doesn´t change the black screen!

Does anyone have any idea how to fix this?

I even tried to desolder the battery first and then the flex cable but it sadly didn´t change the outcome.

Any help would be appreciated, thanks.