boju :
BeggarSifon :
I have same issue with Asus P8P67 Pro and i7-2600K. Tried two GTX 1060 MSI Armor, one 3gb another 6gb. Both work fine in MSI Aegis i7-6700 system. The PSU in P67 system is Corsair CM750M, and it works fine with two R9 380X in crossfire. When i use either GTX 1060 there's boot device error light on the MB. With 2600K i don't have another gpu, only pcie slot. After reconnecting DP, SATA data and power cables system boots and works fine. But the boot device error led on MB is always on, and after shot down or reset problem comes back. Seems to be an issue between new generation GTX and older MB chipsets.
What version bios do you have BeggarSifon?
Due to different uefi standard newer gpus use, updating the bios revision & then clear cmos (clear cmos is a must after updating) is recommended. I'm running a 1070 on a P8P67 delux with bios v1503 (Equivalent to v1502 for your motherboard) and it runs fine. Had memory problems with original bios which then the latest bios being v1503 addressed and didn't need to update for the gpu luckly, but was prepared to and theres been more bios updates since.
boju :
It's recommended to clear cmos properly when updating a bios to reset cmos ram with the new bios version parameters. Theres a good chance your cmos is still using an older version.
i'm sorry, buddy, but we're talking here about new GPU that just came out recently, and motherboard which is over 5 years old. the new uefi standards that you mentioning was probably new 5 years ago. if my bios wouldn't be up to a New uefi standard, then i wouldn't be able to use f(or a few years now) two R9 380Xs crossfired in the fist place. i think there's a problem in the way pcie lanes designed on this board and a new GTX 1060. i'm using it now as i type, but it takes a dozen of cable unplugs, and on/off PSU switching to make it boot, and my second internal hdd not always recognized. whe i put any or both of my NITRO R9 380Xs in this same PC - everything works fine.