Sorry, I flashed that vBIOS with the legacy switch set to CSM, and it bricked. I got it working again by halting PC execution after POST with PSU cables removed from the GPU, connecting the cables during GRUB, and letting Linux recognise the card, and then reflash the backup vBIOS that came shipped (the one giving error 43).
Are you sure that those vBIOS hosted at techpowerups are CSM ones?
I need a factory CSM vBIOS for my GPU. My Motherboard does not have UEFI BIOS, so it can not boot in UEFI mode, at least with the case closed and without the procedure I mentioned.