Upload & Sell: Off
People are discussing CF brands, but it seems to me that you pointed out the problem in your posts: You pointed out that it doesn't work when you format on the PC. And you later pointed out that it works when you format in the 1Dx. Problem solved? Conventional wisdom, which I think is frequently more urban legend than actual wisdom, is to always format the card in the camera.
There might be some glitch the hardware on your PC: trying a different cable and reader would help narrow that down. And just once in a blue moon you can sometimes get different interpretations of what the drive format is supposed to be, which can lead to incompatibilities (and urban legends). If there really is a camera issue here then likely it'll get fixed in a future firmware update to the 1Dx.
Don't let that stop you from getting a better card. But if you have the same problems once the new cards arrive, see above.
And even though it shouldn't matter where I format, I still format cards in-camera, but for these reasons: formatting in camera allows me to be sloppy in removing the card from the computer: I don't need to go through the eject process, just yank it. And I find it faster to access the Format option on the camera than on the PC.