Do you have both EATX connectors, top of motherboard, left of CPU connected ?
Check the all power onto the motherboard is fully seated in to the connectors.
Your Corsair H100i PRO XT has its own backing plate, any chance you put it on backwards ?
should look like this, source https://www.guru3d.com/articles-pages/corsair-h100i-rgb-pro-xt-liquid-cooler-review,1.html
DOCP should be done like this
For the backplate of Corsair HI100, I dont realy understand. Do you want me to turn it back ?
If you set memory clock at 1800 then your memory synced speed will be @ 3600 MHz because it's DDR (double data rate). DDR double data rate means it can transfer data on both rising and falling edges of the clock meaning it can transfer 2 bits of data per clock.
So not really half speed, that's how it works. Memory controller set @1800 x 2 synced for RAM running DDR@3600. Generally keeping the memory controllor synced to DDR speed produces the best results.
Today I've made a fresh windows install. And unfortunatly still the same problem.
I'm now quite sure that it's a ram setting issu. I've check AMD 3950x website and they say it's works with RAM x2 3200 c16 or c18. Many people on forums says that 2x ram is far better than 4x ram for new amd CPU
I'll switch my corsair 4x8 3600 c18 to 2x16 3200 c16. I'll give you news next week.