When you try to add a project with the Project Manager, you get this error message:
Check that the project has a system folder.
Every project has a system folder. The system folder is a hidden folder, so you may not see it in Windows Explorer unless you change your Explorer view options to show hidden folders.
If a project is missing the system folder, you can copy the system folder from another project, and that will work.
Check the permissions on the system folder.
Make sure the user has Full Control, and that all files and folders in the “system” folder inherit those permissions.
If a user does not have access permissions for the system folder, you will get the “invalid project” errors.
Originally posted as a KB article here.