That's either an antivirus problem or a permission problem.
Normally, you should always be able to access your own Documents folder, which contains the Prepar3d Add-ons folder, which will contain the various add-on.xml files so, it's clearly something very wrong in your system.
Try to remove this folder:
Documents\Prepar3d4 Addons\Fsdreamteam Addon Manager
and reinstall GSX using the latest full installer.