Problem: FSX or Prepar3D fails to start, showing error message “Migration Tool has disabled this simulator to prevent corruption of configuration files. To run this simulator, set it as target in Migration Tool.”
Cause: Whenever a simulator is set as target, Migration Tool temporarily disables all other simulators to prevent corruption of configuration files. For example, when FSX → Prepar3D v4 redirection is active, FSX cannot be started to keep FSX from potentially corrupting Prepar3D v4 files. Under certain conditions, simulators do not get re-enabled after redirection is stopped.
Solution: Start Migration Tool, click Edit > Options > Click to view backups. Open an old backup of the simulator that doesn’t start, find subfolder “Root”, and from there, copy fsx.exe or Prepar3D.exe to the main folder of the simulator, accessible through Explore shortcuts in the main window of Migration Tool.
Long-term solution: From version 188.8.131.521, Migration Tool automatically recovers from such situations. See: Migration Tool downloads.