Jay2cent had a defective Asus motherboard problem too. Drove him crazy for weeks. Asus has QC issues. It's difficult to troubleshoot without a test motherboard. I wonder if some of the posts here with the expo problem is cause by a bad board.

You need to win the silicon lottery for 6400 to work on AMD cpus. Update if does work with your new motherboard.

You had in the slot above the pcie x16?

Does it work if you use the older BIOS?

You're using 4x16GB sticks? https://www.newegg.com/corsair-32gb-ddr5-6000/p/N82E16820236909