I'm essentially having a very similar problem to the one in this post:
The biggest difference here is that my error message ends in a different value:
ERROR: Mismatch at offset 0x00000000 - Expected value: 0x4E - Read back: 0x00
I established that the card worked after I installed it and I checked that there was a BIOS listed in GPU-Z. After I installed Prema's vBIOS and restarted (I may have restarted too early), this BIOS version disappeared and says unknown. I have been using different versions of nvflash to try and flash both stock vBIOS rom and Prema's version and started receiving that mismatch at offset error every single time. I don't think my vBIOS chip is faulty but I don't know why I cannot flash any images to it. Currently, my GPU is listed as a Microsoft Basic Display Adapter and nvflash will correctly list my GPU version. Please let me know which log files I should post if it will be helpful.