Dell Precision Tower 3620 Compatible RAM modules

Hi guys

I'm hoping to get some help with RAM modules working for my Dell Precision 3620.

I have 2 3620, both running an Intel E3-1240 v5.

One has 4 8GB RAM module PC4-2400T.  I bought it with 16 GB of RAM (2 * 8 GB) and I added 2 second-hand 8GB modules bought from eBay.  I have had this unit for about 3 years, it runs fine, no problem at all.

A few weeks ago, I bought a second 3620, with 2 8GB PC4-2133P.  No problem at all either.  I decided to increase the RAM, adding 2 8GB modules, but I got the memory failure issue (Orange LED flashing 2 then 7 times).  I assume the modules are faulty.  I bought another set of 16GB modules, second, off eBay, and I got the same error.  The unit doesn't post.  I tried the modules on the other 3620, same issue.  Is it a compatibility issue or I'm having no luck with faulty modules?

The existing modules' reference in the second 3620 is 8GB 2Rx8 PC4-2133P-UB0-11

The first set reference that didn't work is 8GB 1Rx4 PC4-2133P-RC0-11

The second set reference that didn't work either is 16GB 2Rx4 PC4-2133P-RA0-10-MB1

All are ECC modules.

From what I checked, the modules should be compatible with both the CPU and the Dell 3620.  Or am I missing something?

Thanks.