Since installing World Update VI, I have regular been getting the following warning message on launching Flight Simulator. It is annoying, because it pauses the load waiting for a selection and only once "Safe mode" or "normal mode" selected, does it continue to load.
The official support documentation tries to level the blame at 3rd party addons, but in fact this error is common, even with no addons. The trigger that causes this, is a lock file that is created on launch and is meant to be deleted on exit, but often is not.
The file path is:
C:\Users\<user>\AppData\Local\Packages\Microsoft.FlightSimulator_<key>\LocalState
running.lock
Once you exit the simulator, safely check for and delete this file and you will start next time without any annoying error dialog.