Hi @mlohre ,
With the phone switched on, try shining a torch at an angle close to the screen and check if there are any images showing. They will be very faint if they're there.
If they are there, then there is a backlight problem. Either in the display (new display I know but still...) or perhaps with the backlight power supply from the systemboard.
I don't know the phone but with other makes of phone there is a backlight fuse close to the display cable connector on the systemboard.
You may have to use a DMM and start checking components close to the connector and see if any are open circuit. Do this with the power disconnected. If any are open circuit it may be a fuse but it could also be a capacitor and you would have to know how to test them to be sure.
If there are no images showing then the problem may be the systemboard or then again it may be a faulty new display. The only way around this is maybe to temporarily try another known working compatible display to prove it one way or the other.