alt Hacker NewsOdd. I'm using 7 GPUs (8 if you include the onboard AST2500) and haven't had this issue, but these are directly connected to the mainboard
I did disable CSM which used base memory resources needed by REBAR, but aside from that I've 6x Tesla P4s, 1x 750TI, and the AST2500 all happily working together.
My board is a Cloud Star C612 (6x DDR4 variant), 2 PCIE ports bifurcated (so 4 GPUs in total, the others sit on dedicated ports), with a Xeon E5 2696v3 and TBU.
That is because you are using a server Xeon E5 Motherboard at a time when Teslas and Large BARs were becoming a thing whereas he is using a consumer ASUS TUF GAMING X570-PLUS with a PCIe Switch hooked in that makes the MMIO resources from a single PCIe Port suddently becoming gigantic. Unlikely to have ever been tweaked, much less tested, by vendor for that use case.