Booting into white screen

[roadmaus]

Hi,
after following the instructions and trying it with different AHCI drives I am still at loss.

Mac Pro 4.1 flashed to 5.1 (Mid 2010)
macOS 10.13.6
GTX 1070 flashed by MacVidCards_eu

when installed on (different) internal SSDs or HDDs it only works when using the apple startup manager and choosing the UEFI Boot manager at Boot.

If not the mac only boots into a white screen and nothing happens if I press enter twice.

I disabled every program wich could interfere with the boot process but it did not help.

thank you in advance