I have same configuration: intel 5520 and two x5660. It is my first xeon too (for my home hobby).
I wish passthrough very old acard pciscsi scanner, because there are no drivers least win xp 32 bit (please not laugh!). I saw same error and make all known fixes. System do not load after grub update. Find iommu - clear screen.
In the end I tried factory reset for bios (battery and load defaults). Then enable feature vtd and save. Then I powered off energy full and wait for 20-30 minutes. For clear experiment - clear install proxmox 5.1 latest and update grub after install. System is loaded. Find iommu - a lot strings. Start VM - have no error iommu group, but another.
It is very old method of computer repairers. Intel enterprise motherboards have bugs of bios settings saving same cheap noname motherboards!
Tomorrow add blacklist from oficcial instruction and, I think, it is working.
I am sorry simple language (not native for me and sleep now)