Hello, I am experiencing the same issue, but with proxmox as a hypervisor instead of unraid.
I posted this issue on the level1techs forum, https://forum.level1techs.com/t/vfio-passthrough-issues-threadripper-pro-asus-wrx80e-sage/173542
I have updated BIOS & BMC with no luck. I do not yet know of a solution to this.
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Something worthy of note: When I attempt to pass a graphics card through a slot 2-7 that has been bifurcated to 4x4x4x4x, but my graphics card is touching all 16x pins, I get the same error. I verified in my BIOS the top slot is not bifurcated, so I'm wondering if there's some oopsies in the firmware regarding bifurcation of that top slot.