Got the same problem as described above.
First I tried to upgrade from version 1.72 to the Buffalo DD-WRT firmware. But then I got the invalid file error message. I thought that it had something to do with version 1.72, so I tried to upgrade to version 1.74. And it flashed successfully. Then again, I tried to flash the Buffalo DD-WRT firmware, but with no luck. Same error message.
FYI: I have downloaded the firmware a couple of times in case something was corrupt. I have also tried to reset the router to the factory settings and then flash the firmware. Tried to flash the firmware using different browsers and computers. My hardware revision is also B0 C0.
Hope this problem will get solved soon!