Thanks for the tip with IOM.
I will definitely revisit the blob handling (including IOM) on my side because I think that's not the part I can be proud of in my development setup. The chance of something going wrong here is rather high.
When I remove the mca_configure() call, the failure occurs on the next call (LAPIC setup) as well. When I postpone it by wait loops before mca_configure(), it has no effect - so I don't think it's timing related (for details see my original message).
Regarding the stepping, I used A-0 in the past (until February/March) and had memory init issues on the DDR4 variant of the RVP, as you describe, and the reference UEFI could initialize the memory either.
After upgrading to B-0 this issue was solved. Although short after I began with porting coreboot to our design, this MCE error prevented any other progress with coreboot on any of the boards (I continued only with LPDDR4x variant as this is what our design uses and abandoned the DDR4 variant).
Also thanks for the public bug tracker links, that's much easier to follow.