MacPro 1.1 and DELL U2515H

Hi guys,

this will be a long post... ;-)

I've got a new DELL U2515H for my MacPro and G5.
So far, I love the monitor and the picture quality (the only thing I'm missing is an automatic switch between inputs).

But I have a huge problem with my MacPro 1.1 (running Yosemite 10.10.5) which has a flashed XFX Radeon 5770.
The card has 3 outputs (two DVI and a miniDP).

1. Via miniDP (with the included DELL miniDP to DP) I have no signal.
Does one knows if flashed 5770 card have a functional...

MacPro 1.1 and DELL U2515H