FreakyAlleyCat

It sounds as if it doesn't like your ram. 

That is correct; I missed the "mini" bit.

Lian Li o11 Mini only takes SFX power supplies. This one looks like a standard ATX, right?

if you plan on a custom loop, don't get an expensive aio now.
invest that money towards a 5900x and a good cheap air cooler like the gelid phantom.
also the b550m steel legend is fine for the 5800x, but for the 5900x i'd go with the mortar
PCPartPicker Part List Type Item Price CPU AMD Ryzen 9 5900X 3.7 GHz 12-Core Processor €799.00 @ Amazon Deutschland CPU Cooler Gelid Solutions Phantom CPU Cooler €34.99 @ Amazon Deutschland Motherboard MSI MAG B550M MORTAR Micro ATX AM4 Motherboard €146.89 @ Alza Memory Corsair Vengeance RGB Pro 16 GB (2 x 8 GB) DDR4-3600 CL18 Memory €94.90 @ Amazon Deutschland Storage Samsung 970 Evo 1 TB M.2-2280 NVME Solid State Drive Purchased For €0.00 Storage Samsung 860 Evo 2 TB 2.5" Solid State Drive Purchased For €0.00 Video Card Palit GeForce RTX 2070 SUPER 8 GB JetStream Video Card Purchased For €0.00 Case Lian Li O11 Dynamic Mini ATX Mid Tower Case   Power Supply Corsair SF 750 W 80+ Platinum Certified Fully Modular SFX Power Supply €157.69 @ Alternate Case Fan Cooler Master MasterFan MF120R 59 CFM 120 mm Fans 3-Pack €64.98 @ Alternate Case Fan Cooler Master MasterFan MF120R 59 CFM 120 mm Fans 3-Pack €64.98 @ Alternate   Prices include shipping, taxes, rebates, and discounts     Total €1363.43   Generated by PCPartPicker 2020-12-16 15:28 CET+0100    

Your spending alot on a PSU. If you spend 150 - 200 you can afford a 5900x. Also you may want a slightly better mobo.

pro4 should be just fine and is half the price
otherwise looks good except for maybe the GPU might be a small bottleneck

I have a 21:9 and a 16:9 monitor. Honestly you can fit two windows side by side on the 21:9 screen just fine. The only issue is full screen videos. You will have black bars on the side as most if not all internet videos are created for 16:9 monitors. If your ok with that, Id say go ultrawide. 

If you can afford them sure, but which one out of so many is another problem

it doesnt seem to be bothered with either brand of graphics cards, so that's a factor ruled out
 
2060 Super and RX 5700XT are the best value options at this price range (if I'm underestimating prices of the 5700XT, look for the 5700 instead). Performance wise 5700 < 2060S < 5700XT, they'll do 1080p 100-140fps or 1440p 60-80fps in general, tho demanding games like Red Dead 2 stresses GPU so much that the 2060S and 5700 do 90fps at 1080p high settings and 5700XT does 100fps with the same settings. 1440p will tank frame rates further if you dont reduce graphics quality

What is your performance target? Do you only play in 1080p?

Guys, I found a solution to this. I'll post the fix here, in case anyone encounters this thread searching for a solution:
 
I replaced the cmos battery on my mainboard. It seems like the battery was empty, so the mainboard kept forgetting all the changes made to the bios whenever i switched off the power socket overnight. In my case the important changes made to the bios were setting the Sata 5&6 lanes to "M.2" and the boot order, wich also explains why i could always boot to the correct drive from bios. So new battery, fix bios settings, problem gone, awesome!