Proliant DL560 Gen10 Quad Socket config POST stalls during "memory initialization - start"

[SWillis]

Hello!

I'm having issues upgrading an older existing server outfitted it with 4x Xeon 8180's and a modest 256GB of DDR4 2666MHz RDIMM memory with two primary channels populated per-socket in slots 8 and 10 (CH1, CH2).

Each CPU has been verfied to complete POST and get to boot individually and in pairs (P1, P2) or (P3, P4). Main board only OR Mezzanine board only, but never all four nor two across (P1, P3) (P2, P4).

What I've tried so far

• Updated Firmware, to latest ILO, BIOS, UEFI
• Tested minimal configuration using the dimm slots following the correct population order referenced in the documentation.
• All DIMMS verfied to work together and individually.
• All Processors verified to work.
• Processor Pairs verfied to work.

What is very strange about this is all the components are working in various configurations EXCEPT when it's setup as a quad socket.

I'm using 4x 800W power supplies 720479-b21 (Spare # 754381-001) in 4+0 mode. Could that be the problem?

There are no errors or messages that indicate a problem and I've tried leaving it for hours and it doesn't progress past
"memory initialization - start" to "memory initialization - complete" so I'm lost as to what I could possible look for to research or fix the problem since all the components work individually... I'm not sure what I can replace without running into the same problem.

Thank you for reading! Please help me, This server was very expensive.