Error 78 screen not detected

[salapala]

My setup:
Aorus master z390
i9 9900k
Asus Strix RTX 2080 Ti OC
32gb trident z 3200mhz
960 evo m.2 nvme 512gb
Asus 27" ROG PG279Q

Issue:
Error 78 on motherboard on boot, screen not detected.
This started after flashing my bios to the latest version - f8b

Things I've tried:
- Hdmi cable, displayport cable (switched screen output correctly)
- Made sure that the dp cable is properly connected to gpu & screen
- Clearing CMOS
- Removing and re-connecting cables for motherboard & gpu
- Removing all usb connected devices before boot
- Switching ram slots and/or removing ram
- Booting without m.2 drive (this gives error A2)
- Using different card (GTX 980 Ti)
- Using different monitor and dvi connector (Benq xl2411z)

Previously when I've run into a similar issue it was caused by "secure boot" in bios but now I can't even access it because both of the screens I tried are dark.

 

[alexoiu]

Does the board have a Dual BIOS switch? If yes, try it.

 

[salapala]

Yes, forgot to mention that I've tried that as well. Same problem.

 

[alexoiu]

If https://www.overclockers.com/forums/showthread.php/697533-GUIDE-Forcing-backup-BIOS-on-Gigabyte-motherboards
still not helping, then I would return the board, to be replaced.

 

[salapala]

Other things tested by now:
- Switching PCIe lanes with both GPUs
- Tried #1 & #2 methods from the link above - didn't try #3 as I have no clue what I'm doing.
- Hdmi to my tv (probably a pointless test - getting desperate)

 

[alexoiu]

I would return the board, to be replaced, if under warranty.