Upgraded my motherboard to latest 4007 bios with an 3800x in place. Been using that system for a year w.o. any issues.
Works flawless after upgrade.
Install 5800x and the RAM-led lights up.
The memory sticks is OK and both of them works single and together in A1 + A2.
Place in B1 and/or B2 and the computer does not get past POST and the RAM-led lights up.
Both slots is powered (the LED on the memory stick lights up) and both slots work with the 3800x in place.
Seen a few others with same issue with this motherboard/BIOS.
Guess i just have to wait for new BIOS to see if that fixes the problem.