X10SRi-F — memory channels A/B completely dead (C/D fine), diagnosis and options?

Hi everyone,

Home-lab server that stopped detecting RAM in half its slots. I've done a fair amount of testing and would like a second opinion on both the diagnosis and my options.

Hardware:

Board: Supermicro X10SRi-F (BIOS 3.4, the latest version)

CPU: Xeon E5-2687W v4

RAM: 4× Samsung 8GB 2Rx8 RDIMM ECC 2133, plus attempts with Micron 16GB 2Rx8 RDIMM ECC 2400

Case: Phanteks Enthoo Pro

Symptom: 4 of 8 slots work. Channels A and B don't detect any module. C and D work perfectly.

What I've tested (in order):

dmidecode -t 17: slots A/B show "No Module Installed" even with modules seated; C/D detect their modules fine.

IPMI SEL: repeated "Uncorrectable memory component found (DIMMA1 / DIMMB1)".

2 new Micron 16GB sticks failed in A1 and B1.

The decisive test: a known-good stick (works in C/D) stays undetected in A1 and B1. The fault stays with the slots, not the stick.

BIOS already up to date (3.4 = latest).

Pulled the CPU and inspected the socket: to my eye it looks fine, I don't see anything obvious.

My questions:

Does the "A/B channels dead on the board side" diagnosis look solid, or did I miss a path?

What can kill an entire channel pair on the board side while leaving C/D intact? Socket pins, memory VRM, controller, broken trace… where would you look?

Any additional tests to run before concluding? (BIOS Memory RAS / topology display, CMOS reset, CPU reseat, socket cleaning, etc.)

On options: is repair realistic? If not, replacement — would you go for another X10SRi-F/X10SRL-F ?