Error code 16 - X399 Taichi - TR1900x - Corsair

Hello everybody,

my computer consists of these (main) components:
- ASRock X399 Taichi AMD X399 So.TR4 Quad Channel DDR4 ATX Retail
- AMD Ryzen Threadripper 1900X 8x 3.80GHz So.TR4 WOF
- 32GB (4x 8192MB) Corsair Vengeance RGB PRO schwarz DDR4-3000 DIMM CL15-17-17-35 Quad Kit


The problem:
- Upon powering up, I get an error code 16. Don't even get to BIOS
- More telling is the fact that RAM installed in slots D2 & C2 lights up when connecting the motherboard with power (don't even need to start computer). RAM installed in slots A2 & B2 NEVER lights up!

What I tried:
- I switched RAMs around and reset CMOS after each switch. The RAMs themselves (or my installation method) don't seem to be the problem, because each one works fine in slot D2 & C2. The DIMM slots are the issue
- During this procedure I believe I also got an error code of 10 at one time, probably when DIMM slots C1,C2,D1,D2 were all empty. Don't think that this is helpful information.
- I remounted the CPU and looked for bent pins. To my surprise all socket pints were slightly bent (expected straight ones), but I am pretty certain that this is expected behavior. Didn't see anything unusual.
- My understanding is that it can either be an issue from the motherboard or from the CPU memory controller. Could I narrow it down somehow without access to a second compatible CPU / motherboard?

Any other ideas what I could try? Thanks in advance!

TLDR: Most likely bad RAM position, same as http://forum.asrock.com/forum_posts.asp?TID=11152&title=x399-taichi-tr-1920x-dr-debug-code-10-16

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Soo, sent the mainboard back, got a replacement (for free). Put everything together again and got the exact same kind of error and was close to sending the CPU back.

However, then I double checked the RAM again by putting all four into other positions. Again, RAM in C2 and D2 light up, in A2 and B2 not. Despite that, I "magnically" got into the Bios, where it showed me only one of the four RAMS (namely in the D2 slot). To my surprise, the RAM in D2 sat indeed slightly lower then the RAM in C2, even though both had the lights on!

Thus, I reinserted all other RAMS with lots more wiggling and similar amount pressure to get the same seating. Now, I get into BIOS and all four RAMS are shown. I assume it's gonna work now, though I haven't installed an OS yet and DrDebug is showing lots of numbers during startup.

Crazy. This was not my first build and I reinserted the RAM dozens of times with quite some pressure on both sides and the resulting click. But apparently this wasn't enough. I don't understand (1) why the insertion of the RAM can be so error prone (apparently not only for me) and (2) why the correct seating isn't stressed more in the manuals. In order to help others, here are two pictures of the wrong and correct seating:

Right: https://drive.google.com/file/d/1WTAJjobOmY7DC95Gt_M-SPgC2AD7En5e/view?usp=sharing

Wrong: https://drive.google.com/file/d/1SHdTLmBOA6qY1cDX-kpo83TyRItVcq-g/view?usp=sharing



P.s.: I omitted one intermediate step: After having only the RAM in slot D2 correct, I got similar symptoms as the following thread http://forum.asrock.com/forum_posts.asp?TID=9937&title=asrock-x399m-taichi-dr-debug-random-codes . Therefore, I also switched my GPU position from PCI4 to PCI1. I thought that this was just a recommendation, but seems like a hard boundary.