pci 64bit is already enabled, as is >4gb decoding and the correct MMIOH settings.
That was stopping 2 card from working, it was posting but the driver would calltrace when nvidia-smi was run.
Advanced->PCIe/PCI/PnP configuration-> Above 4G Decoding = Enabled
Advanced->PCIe/PCI/PnP Configuration->MMIOH Base = 256G
Advanced->PCIe/PCI/PnP Configuration->MMIO High Size = 128G
There should be enough pcie map space between those for the cards... but maybe another device is getting in the way and using some of the BAR space?
@Patrick might try increasing that MMIOH Base number.
Fun fact, I was having the same issue on my Tesla K40 in my SL270 gen8, I thought the card was just dead as it was sitting around for years.
When I saw he had the same error, I helped track his down so that I could use the same fix on mine. I just had to enable pci 64bit support but it is in a lovely hidden menu that you press Ctrl+a to unveil...I was not amused.