Is the problem happening on every airplane you customize ? If yes, that might be caused by another software interfering, like an antivirus, which is mistakenly prevent GSX to write into its own folder inside your user account. Or, it might be a problem with your user account.
- Configure the antivirus to exclude the whole Addon Manager folder and this folder too:
%APPDATA%\Virtuali
- If that doesn't fix it, REMOVE the %APPDATA%\Virtuali and REINSTALL GSX. Be careful that, if you do this, you'll lose every customization you made or downloaded for airplanes and airports so, maybe it might be best to do a backup of this folder first, if you have files there you want to keep.