T3610 suspected faulty dimm SLOT_1. Any possible other causes/suggested workarounds?

The issue is as follows:

When no memory is installed, I get error LEDs 3+4. (Essentially means no memory detected)

When a dimm is installed in SLOT_1, I also get error LEDs 3+4.

When a dimm (or multiple dimms) are installed in SLOT_2-8, I get error LEDs 1+3. (memory detected, but invalid configuration).

I have a number of kits of memory; ECC, RDIMM, UDIMM, both 12800 & 10600, all behave the same in this machine, and several of them have been tested successfully in other machines.

So I'm pretty certain the problem is NOT with the RAM.

I've reseated the CPU (there were no bent pins).

I've reset the CMOS (both by removing the battery for 10mins, and by using the RTCRST jumper for 10s).

I've tried changing the CMOS battery too.

I've cleaned out all of the memory slots.

The only thing I've not done is to switch the CPU, but unfortunately I don't have another LGA2011 CPU to substitute.

I strongly suspect something is wrong with dimm SLOT_1; the socket itself, the trace to the CPU, or perhaps a fault in the CPU's memory controller. (The CPU is an E5-1620 v2).

Is this a known hardware issue? Or is there some kind of misconfiguration that I'm not properly clearing?

Is there a workaround, or will the machine only boot if it recognizes memory in SLOT_1?