After some experiments and reading on this forum I found out that the stuck black screen on loading was caused by my headset. It was connected to the computer (phone connectors in the chassi). When I unplugged them and tried to run MSFS again - it started really smooth.
So I believed I had fixed it, and plugged the headset in to the computer again and it all worked for a short while. However the menu started to stutter and MSFS slowly degraded until after a flight it all went black again.
So I then tried to plug the headset into my new monitor (which is connected to the computer via one display port) and the problem is gone.
I huess MSFS has a serous bug regarding computers with other sound-source than the HDMI/Display port. My computer has both Nvidia sound and Realtek sound (jacks in the chassi).
Monitor: Samsung 49" Odyssey C49G95
Screen res: 5120x1440
Operating System: Windows 10 Home 64-bit (10.0, Build 18362) (18362.19h1_release.190318-1202)
Processor: AMD Ryzen 9 3900X 12-Core Processor (24 CPUs), ~4.0GHz
GPU: GeForce RTX 2080 Ti, (ASUS ROG STRIX GAMING OC) 11Gb
DirectX Version: DirectX 12
Installed on 500 GB, (Samsung 970 EVO PLUS)