Hi folks,
after a lengthy bisection run I was able to nail down the commit which is causing the MC4 errors in the configurations I tested. I reverted the commit on top of current master, and now my KGPE-D16 fully works without MC4 errors in both 1-CPU-package und 2-CPU-package configurations - that is, with all 16 memory slots populated with the 8GB DDR3L Samsung RDIMMs (PC12800 variant) mentioned in my initial post.
However, the issue with memory clock (being reported at) 667MHz in dmidecode still remains.
I submitted a request for reverting the causing commit [1].
Cheers, Daniel