Yes, the 4 of them were seated properly, I have tested them on a friend's rig and they work. So it is neither a memory error nor a seating issue.
My hp is OMEN X 900-201nx
I have updated the BIOS following HP's directions. And I tried the RAM again and it gave me the same three short beeps I have nightmares about.
I have tried contacting HP by phone but it was of no help. I will try their forums.
In the meantime, any other options other than replacing the whole motherboard? and pretty much the whole PC since if I change the motherboard I need to change my CPU (I9-7920x)
I would first put the original RAM back into the system first, so you are able to get into the BIOS. If you updated the BIOS correctly, it should say the BIOS revision somewhere within the BIOS information page, etc.
It sounds like the motherboard doesn't like the 16GB density memory sticks...but I don't know why...when HP specifies up to 64GB...but they gimp it?
The motherboard looks like "standard" ATX form factory for the most part.
You should, be able to just swap the motherboard out, and you are good to go.
Keep in mind, since the computer case / chassis accepts micro-ATX, are you limited to those sizes, BECAUSE of the case / chassis. It looks like you can basically re-use everything else in that system (e.g. power supply, CPU, graphics card, etc, etc).
LGA-2066 is STILL Intel's CURRENT flagship 'Extreme' series.
You can re-use that CPU in a retail LGA-2066 supported motherboard.