Can you downgrade to previous BIOS 0801?
After receiving my VIII Gene motherboard from RMA with BIOS 3703 (RMA due to code 00, T/S GPU Tweak II version 1.6.0.5, still using GPU Tweak II version 1.4.7.3) I experienced a similar problem. When I tried to downgrade the BIOS from 3703 to an earlier BIOS 2202 the computer would not boot, it would only boot with BIOS 3703 installed.
Anyway, I only used the USB BIOS Flashback like your motherboard manual on page 2-13, but for the VIII Gene. And it still did not work. I could only install BIOS 3703 for the motherboard to work.
So, I had to reprogram the BIOS chip and it has been working flawlessly for an upgrade/downgrade for BIOS 0701 to BIOS 3801 (new release), but I’m still using BIOS 2202 (I7-6700K).
Anyway, if you cannot downgrade to a previous version from BIOS 0906 using the instructions in your manual on page 2-13 and nothing else works but BIOS 0906, you may have a faulty/corrupted BIOS chip (BIOS chip nowadays being soldered on motherboard). Otherwise, if you can downgrade your BIOS, I’m at a lost.