Dell Precision 7910 adding a second CPU

I added a second matching Zeon E5-2687W. Running the most recent BIOS from Dell. The machine had 64 GB of RAM in 8X8GB DIMMs that worked when only CPU1 was installed. The RAM populated all the CPU1 DIMM1-8 Slots. Now, adding the second processor, I moved the RAM to match CPU1 DIMM1-4 and CPU2 DIMM1-4. BIOS recognizes everything. A Proxmox install that has worked fine until adding the second CPU hangs on boot at creating the RAM drive and never finishes booting.
To troubleshoot,
- I removed all the RAM except CPU1 DIMM1 and CPU2 DIMM1 I got the same symptoms: no boot after Proxmox starts creating the RAM drive.
- I removed the second CPU and placed all the RAM back in CPU1 DIMM1-8, it boots fine.
- I exchanged the processors (old and new) into slot 1, and it boots fine; this leads me to believe both processors are good.

- I have tried new loads of Debian, Ubuntu and Windows 11.

I'm sure the RAM is good as it all works when CPU1 is the only processor present and all 64GB RAM is in CPU1 DIMM slots.

I'm stumped. From what I have read, the processors meet documented specs, and the processors match. The RAM meets specs, I believe it is slotted correctly with both processors in. Does anyone have any Ideas?