This afternoon I finally got the earthmover (...) to carve out some room at the bench, enough to have two of these behemoths open side by side. Not sure I'll be able to get the car we usually garage inside the garage tonight...
So, I was able to swap modules from the known good unit to the issue unit such that I'd isolate the issue. And this far tests are a bit confusing. Per SM (p.4-14), given the error message, A8, A9, and A4 are prime candidates for issues. A4 and A8 A9 seem to change nothing, essentially, so I'm deeming them good. Swapping A9 A8 doesn't fix the issue, but instead returns a different error. Actually, errorS! Initially, there was something about, I think, "inside guard" (it was one time and I didn't write it down immediately), and now I'm consistently getting "OUTPUT LIMIT FAULT. OUTPUT OVER VOLTAGE." But, this was upon me asking it to output 10V, and I didn't see it outputting more, so that's a bit weird. It seems there's some false flag errors triggering or something.
I'll dig down deeper, but if anyone sees any sense in the above, I'm all ears. All I can conclude this far is that the issue may have to do with A9, though a known good module didn't fix it. For anyone asking, the module in both units is a "5440A" module, so I don't suspect any updates to the "B" unit modules make the swapped "A" unit unfit.