Hi,
I carefully read all answers from this topic.
I tried all workaround as Microsoft still does not provide any patch regarding this problem.
I also have extended ASCII character in my Windows profile and does not want to explicitely create new user only for the game.
I confirm manually delete old custom flight files solves the problem, before launching the game.
I created the workaround automation, if any user does not want to do this operation manually.
A shortcut to launch FS2020 is supposed to exist on your desktop.
Create a new batch file, like launch_fs2020.bat and put this content inside:
DEL %APPDATA%\..\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\MISSIONS\Custom\CustomFlight\CustomFlight.FLT
DEL %APPDATA%\..\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\MISSIONS\Custom\CustomFlight\CUSTOMFLIGHT.PLN
DEL %APPDATA%\..\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\MISSIONS\Custom\CustomFlight\CustomFlight.SPB
“C:\Users\%username%\Desktop\Microsoft Flight Simulator.lnk”
Launch the .bat file to run a clean FS2020 instance.
Hope Microsoft will create the patch, which is easy to fix I think, for all user with potential extended ASCII characters in their names (French, Spanish, Norvegian, German, Russian, Japanese, etc. Lot of people potentially impacted).