After a few hours of searching on the web, I finally found the solution, in a post from 2020, known problem.
for me it’s working perfectly for now and fs2024 in ultra super fluid and responsive tonight.
SOLVED!!!
It’s known that the problem appears when using non-english characters in windows user name because the sim can’t find the custom flight folder when you are setting up a new flight.
I’ve resolved the problem activating, in my Windows 10 (build 20H2), the “Beta version: Use UTF-8 for language compatibility” checkbox.
To do that, you have to go to “Control Panel” → “Clock and Region” → “Region” → “Administrative” Tab → “Change system regional configuration…” → and check the “Beta version: Use UTF-8 for language compatibility” checkbox.
After that the problem doesn’t appear anymore and you can set up a new flight without any problem.
Sorry, I couldn’t check that in other Windows versions.