Ok, so you have the latest BIOS version, the memory is definitely fully compatible and the motherboard was released in 2008 making it about 11 years old. I'd say you have a bad motherboard since the memory works in another slot, so it's not the memory itself.
In most cases 5-7 years is a pretty good run for a motherboard. 8-9 years is fairly uncommon and 11 years is rather exceptional. I think you need to consider an upgrade or a replacement motherboard, and finding one of those that isn't equally antiquated is probably next to impossible. It's very doubtful you'll find any new old stock that isn't more expensive than a brand new, much newer generation motherboard.
I don't really see any scenario where it's NOT the motherboard to blame.