Very likely an incompatibility.
The old board supports PCIe 2.0 on its x16 slot. The graphics card is likely PCIe 3.0 and claims backward compatibility.
The old board would not know what the future holds, so the manufacturer of the card would have the responsibility to ensure 2.0 compatibility..
This issue has come up many times before. You might see if there is a firmware update for your card, or try and find a card that is really compatible. Perhaps an older card will work for you.
Current card I am using is also PCIe 3.0 and is working happily than ever before, so it can't be that ! Pcie 3.0 is backwards compatible with pcie 2.0. I agree on the apparent -most likely- incompatibility between DX58SO2 and Sapphire Radeon Rx560, just not about pci . I am genuinely wondering about the other current graphic cards, for example, does a Gtx 1060 works with Intel DX58SO2 ?
Let's hope someone answers !
Just to clarify, I believe it is your particular Sapphire card that is NOT fully backward compatible. If you do have a card that is working and is PCIe 3.0, is it the same manufacturer and model, or not?
I have seen many of these posts, and many of those with the Sapphire card, that have the same issue as you.
Personally, I would not want to run a nice, new PCIe 3.0 card on such an old board. While you may find a card that does work, you will still be running in a degraded mode (PCIe 2.0).
Anyway, good luck on your quest.