TestStand stores configuration settings in .ini files in the <TestStand Application Data>\Cfg and <TestStand Application Data>\Cfg\ModelPlugins directories.
Be sure there were no changes applied to List of users, User names, passwords, privileges and other information (Managing Users ) via User Manager Window.
All information related to users is located in UserInterfaces folder. Open TestStand(your version) folder using File Explorer and make sure UserInterfaces file exists.
If the folder is missing please reinstall TestStand from your machine.