@bogikornel We had a very old XO built from sources, which worked fine. One day we decided to deploy a new VM and migrate to a new XO built from sources, and immediately ran into problems. All tho it was resolved, after switching machines from HP to Cisco, which makes us believe it has something to do with the NIC, driver or firmware.

So I ordered some mini DP dummy plugs and took a look at this further today. First thought...Just load Windows on the bare metal and see what the card does. No problem. Loaded Win10, loaded the latest drivers, PCIe Gen4x8 lanes. Sweet. At least that works. Put the XCP-ng drive back in, booted up, passed-through the GPU to a Win11 guest, updated the drivers, PCIe Gen1x1. Ultimately I need to install a Linux distro with a really current kernel on the bare metal and see how it sees the GPU. That might be a good project for tomorrow. I think this just comes down to a combination of too new, not well supported hardware with some quirks and somewhat dated code and support in XCP-ng (ReBAR support in guests for example). @redakula Is that GPU-z screenshot from a Windows guest VM with the A310 passed through?