[wpm]

Depending on your server's CPU sockets, reseating the CPU might help if they were ever swapped before (esp if you got the server used). I have a marginally bad RAM slot on one of my Supermicro X11 boards because my dumb ass dropped the CPU into the socket and bent a few pins. I bent them back, but you can only do so much and the furthest RAM slot won't work without throwing errors daily if I run it at 2400MHz.

[etbe]

I have a HP Z640 that only has 3/4 DIMM slots working. Reseating the CPU didn't help and the same CPU works perfectly in another Z640. I never found out the cause. It was sold as "unknown condition for parts" so $50 for a server that works perfectly apart from one DIMM slot is still a good deal.