I got a second hand rx 480 a few days ago, and it was folding great. I swapped it into a different machine, and now it instantly downloads a wu, claims bad wu, sends it back, this happens 5 or 6 times then it comes back failed. It doesnt even get to the point where the card is being put under a load. I swapped card back into machine it was working fine in, as well as a 3rd machine. Same story. The error is Exeption Access violation.
If this is in Windows, I've never had that happen before. I did have a similar issue with a 480 in Linux, but that was when the new Linux drivers were pretty immature.
Have the Bios been flashed on the card? If so, I would reset them to factory and then clean and reinstall drivers. Since it is happening in more than one machine, it could be a hardware issue. How does the card handle other loads like Unigine, 3D Mark, etc?