Hi everyone, first post on this forum after lurking a bit, including on the issue described below.

I've had a PS/1 2133 (486SX-25, 2MB 72pin SIMM, 85MB HDD) for a couple years now, and it has worked without a hitch.
Recently I got a couple upgrades for it - a 3COM Ethernet card and an ALS100+, both working fine. But what didn't work is the RAM upgrade - I got 5 confirmed working sticks, and all gave me the same 201 error:

...but it did show the total amount of RAM that should result if both sticks were "good":

What's weird is that this error only occurrs when I insert the working stick in bank 0 and the "failing" sticks in bank 1. With just the "failing" stick(s) in any bank, the system does not POST at all with 1 long and 1 short beep. With just the working stick everything is totally fine. Thinking it's just something IBM did for "not approved" sticks I ran the advanced diagnostics utility, which was even more ambiguous than the POST error, giving me a dry "Replace SIMM in bank 1" message.

Stumped, I looked at the HW maintenance manual for at least a smidge of help, but did not really get anywhere. I didn't find my board FRU in there either (52G2928), with the closest being 52G2968. Specs matched too. I did find a picture of a board with the same layout but green substrate on The Retro Web, but the FRU is different - 65G3704, which also doesn't exist in the HW maintenance manual.
In case anyone can identify the board further, I will attach the pic of the board:

SIMM in bank 0 is the working SIMM, the one in bank 1 is "failing".

So, I'm wondering - does my board only accept some sort of proprietary/pre-approved sticks or could that be a hardware issue on the board itself?