This is from the MS Troubleshooter.
DELETE STEAM CLOUD SAVE (STEAM VERSION)
WARNING: Doing so will delete your logbook permanently.
If the crash happens upon loading:
- Close Microsoft Flight Simulator
- Right-click on the Microsoft Flight Simulator icon
- Go to Properties
- Uncheck the “Keep Game saves in the cloud for Microsoft Flight Simulator” box
- Go C:\Program Files (x86)\Steam\userdata
- Find the folder corresponding to your profile and delete the folder Steam\userdata\usernumber)\1250410
- Restart Microsoft Flight Simulator