You could try running MemTest86
on basic "optimised defaults" (no tweaks or changes to BIOS settings beyond those necessary to boot it from a USB stick). It may even be built into your BIOS, as ASUS include it in some of them (no USB stick necessary, although you may still want to try the latest version from USB).
If it does find a problem, swapping the memory around can give a clue to where the problem is. If the problem moves with the RAM stick, it's probably the stick. If the problem doesn't move when you swap them, it's probably the board or socket. Also test with just one stick, then just the other stick. Basically try all the combinations / permutations of sticks and sockets to see if anything either finds a problem or makes it work. Make sure you are using the correct single/pair of sockets out of the 4 sockets on your board (the manual will tell you which socket to use for a single stick, or which pair of sockets for two sticks).
Also, get a bright flashlight and carefully inspect the sockets looking for any damage or something odd about one of the contacts inside the socket. DDR sockets are fairly robust, but there's mechanical forces at play and little gold fingers that need to be properly aligned. Without applying significant force, check that the sockets seem to be firmly attached to the board (no real movement in them under light prodding). Look for any sort of contamination / debris that found its way into the sockets.
Is it possible that you physically disturbed your CPU while swapping memory around? If so, remove it from the board, inspect both socket and CPU, reinstall carefully, etc. (Those tiny pins are very fragile, don't prod at them, only visually inspect them.) Depending on the thermal paste you have and how long it has been on there, it could have dried out and the work done next to it broke or degraded the thermal bond to the cooler. Check both the CPU and cooler are properly and securely in place. I don't particularly suspect the CPU from what you wrote, but it's next to where you were working and what is on the far end of those tiny contacts inside the RAM slots, so kinda the next thing to check.