had the same problem for 3 days. Figured out the problem for me was with the Game Bar update failing to update completely.
Rather than uninstall MSFS2020, I uninstaller the Game Bar, and after 4 0r 5 attempts to reinstall, all unsuccessful, I just left it nagging and went away for an hour. When I came back in great frustration, and it was still hanging with an ERROR message, I shut the reinstall attempt down and immediately got a message saying the reinstall was complete. WTF!!!
But it had reinstalled. So MSFS2020 opened normally, and I progressively added my stuff back to the Community folder, and all has been OK for the last 3 days. But I expect further problems. Besides this issue, I have done 3 complete reinstalls of MSFS2020 in the last 6 months. I now have the program guts installed on my SSD "D" drive, and that drive is cloned to a backup HDD. So hopefully I won't have to download the whole thing again. Maybe just the small exe that goes on the C drive.
Shame really, such a brilliant program, spoiled by such a complicated and arcane delivery system. Cant live with the problems, nor do without the program.