Precision 7810 single RAM slot not working

Hello,

I just tried to upgrade to 128 GB RAM and get the 2-7 code upon boot. All Slots work fine except CPU 1-4, I tried pressurized air and a vacuum to remove invisible dust, to no avail.

The WS was purchased as refurbished in November 2022, so I figure the CMOS battery must have been replaced and the PSU checked.
Is there a way to diagnose what's going on and are replacement parts availabe? 128 GB would cover my needs, anything below is problematic.