If the old GTX970 worked and the GTX1080 does not, you have a driver issue or compatibility issue with the GTX1080.
Let me restate that: If the old GTX970 card worked fine but the GTX1080 card does not work until Windows, then there is an incompatibility between this GTX1080 card and the BIOS. Unfortunately, since this board has been end-of-lifed (as a result of Intel exiting the desktop boards business), there is little (if any) chance of this BIOS issue being fixed.
Sorry to be bearer of bad news (Doc and I get to do this a lot)...
P.S. You are running on the latest available BIOS release, right? If not, put the old card back in the system, update the BIOS to the latest and then see if the new card will work (unfortunately, there is still nothing that can be done about it if you find that it doesn't)...
Also, it may be that the firmware on the GTX1080 needs to be updated, Check with the card manufacturer.
Just a thought.
Hi guys, thanks for your answers. My problem is solved.
I connected a second monitor via HDMI. The second monitor shows the Bios screen. I disabled the UEFI boot and saved the settings. Now the bios screen / post works with the display port connected monitor only.