Check to see if the cooler has bitten into the PCB. Have seen cases where people didn't mount the cooler correctly (or left out the washers), resulting in trace damage.

I have a similar issue and I am about the RMA my Maximus X Hero (WI-FI AC) for the second time. Everything worked perfectly with my 4-stick 32GB Corsair Vengeance LPX 4000 memory kit up to 4200 CL17 for months. When I flashed from 1101 to 1602 everything immediately went haywire and now I can only use 4 sticks up to 3800. But, 2 sticks work fine up to 4200 CL19 in either pair of DIMM slots. Downgrading to 1101 or earlier BIOS using a normal flash or BIOS Flashback does not restore previous functionality. The Intel ME firmware version does not downgrade along with the BIOS version.

What is different in my situation is slowing down the RAM to 3800MHz corrects the problem with 4 sticks installed and removing the water block to relieve pressure on the CPU doesn't change anything. Booting at 4000 or 4200 with 4 sticks is not an issue either, only problems involving Windows file corruption, Windows Boot Manager not being found, and random BSOD, with 4-sticks installed running higher than 3800MHz.

The only thing I can identify that caused the system behavior change is the new Intel ME included with 1602. I skipped over BIOS 1401 (which also had the newer ME version like 1602). The only reason I bothered flashing 1602 was the intent to upgrade to 9900K, and now I regret updating the BIOS.

My normal approach to drivers and firmware is "if it ain't broke, don't fix it" and wish I had stuck with my normal M.O. now. The latest is often not the greatest where drivers and firmware are concerned. Hindsight is always 20/20, LOL.

tuoc65 wrote:
OK well, guess bent socket is what it is. I can get the system to post with 4 DIMMs if I take the CPU cooler off.

I tried putting the CPU cooler on and held it down with my finger and no screws, it doesn't boot. Love it.

This does sound like a hardware issue in your case.