I had the same issue: code 78 on an Asrock Rack EPYCD8-2T with a 7551 from eBay. Turns out the issue was that the CPU was pulled from a Dell system, which burn a key into the CPU such that it will only boot a Dell-signed BIOS. This makes the CPU unusable in any non-Dell system.
Thanks for the e-waste, Dell.
More details here:
https://downloads.dell.com/manuals/common/security_poweredge_amd_epyc_gen2.pdf
How did you determine they were pulled from a Dell?
I am having similar issues to OP. I have brand new H11DSi-NT, 2 Epyc 32 Core 7571. After building out hardware, the server powers on and I get green flashing heartbeat LED light and solid green LED, both indicating that functions are normal. However, I get nothing showing on confirmed working monitor and confirmed working vga cable. I just get a black screen, no cursor, nothing. It took me a long time to figure out how to IPMI in but I was finally able to. Initial searches for default username and password did not help and I had to dig relatively deep to find that the password can no longer be a default password due to some law passed in California, and that the actual password is on a tiny sticker on the mobo. Upon entering the IPMI screen, the first thing I notice is that the hardware tree shows completely different hardware than I actually have. I assume this from the manufacturer's testing, but I really have no idea. It is showing (A) 2 16 core CPUs, and (B) 8 gig RAM sticks in every slot, both of which are not present. Also, there is nothing showing in the video preview section of the IPMI screen.
I have now changed the BIOS multiple times both trying older and recent versions. I have cleared the CMOS removing (and changing) the battery and "jumping" the touch pads using a small piece of metal. I have tried three different RAM sticks together, separately in all the various DIMM slots. No matter what I do, it always show the same hardware config which, even if the mobo can't read the actual inserted CPUs, I would have thought the old/tested hardware config would get reset or removed once I updated bios, cleared CMOS.
I have also tried swapping the CPUs positions and running them individually with the 8 pin power supplies removed for the empty CPU slot.
I mostly get POST code 'ff' from IPMI unless I insert a certain RAM stick into slot B1 in which case I get POST code '16'.
I've also tried changing the VGA jumper.
Since the only two piece of equipment that were not brand new or already verified as working( by me) were the CPUs, I am assuming these are the issue. I think it's very likely that they are either BOTH faulty or (since finding this post) pulled from Dell machines and therefor cannot be used on a Supermicro board.
Any feedback/help would be greatly appreciated.