Got this brand new 12400 CPU to try some BCLK OC but got hit with the QCODE 55 error on an ASUS Z690 HERO motherboard. Did some digging and turns out to be faulty CPU IMC. Saw a guy on Overclock.net with the same issue. The slot on the far right (DIMM_B2) simply does not work.
Latest BIOS > No luck
Corsair DDR5 > No luck
Kingston DDR5 > No luck
XMP 1 enabled with just 1 stick in DIMM_A2 > Runs OK
Insert the other stick to DIMM_B2 > Fails at boot
MRC Fast Boot Disabled > No luck
Trying to get another 12400 right now and planning to shoot a video about the issue.