I have a feeling this might be related to the IOMMU groups. When configuring PCI passthrough, it is important to understand the IOMMU groups; if you passthrough only some of the devices in the group rather than all of them, such problems may occur.
I found someone on another forum who has the same board as you and posted about IOMMU groups, but that grouping seems a bit odd…
Also, even when devices are grouped in the ideal configuration of “one device per group”, some problematic boards (though it’s unclear whether this is Xen or a board issue) may crash when performing passthrough (as is the case with the board I have).
So, If you want PCI passthrough to work as expected, you should buy a model that has good reputation on relevant forums. (However, since many of those reviews assume the use of KVM(Proxmox), it remains to be seen whether they will work on Qubes OS using Xen…)