Datapoint:
I bought a Super Micro X13SAE-F board, and 2x32GB DDR5 ECC UDIMMs (Kingston Technology 32GB 4800MT/S DDR5 ECC CL40 DIMM2RX8 Hynix M - P/N: KSM48E40BD8KM-32HM).
CPU (12600k) is known to be good. Worked before and after being installed in the X13SAE board. System will not post with the ECC RAM installed. Tried each DIMM in each slot, individually. I’ve opened a ticket with Super Micro, and am awaiting their response. The only ECC RAM on their QVL is for a DDR4 product, which is obviously wrong. Latest BIOS is installed. I wouldn’t be surprised if the product doesn’t actually support DDR5 ECC UDIMMs.