When you click on "Save Setting", dbFront requests the credentials of a Server Administrator.
Once you enter the credentials dbFront "impersonates" that administrative user so that it can save the necessary settings in the application config file.
This error message means that the user you specified no longer has the necessary access to replace the configuration file in that folder. You can validate this by opening Notepad and attempting to create a file in the folder 'C:\Program Files (x86)\dbFront\'.
Once solution is to directly give your Administrative user "Full Access" to the folder 'C:\Program Files (x86)\dbFront\'.
One item that may be confusing is that the specified user may already be a member of an Administrator group which has access to that folder and therefore should logically already have the necessary access.
The special groups ALL APPLICATION PACKAGES and ALL RESTRICTED APPLICATION PACKAGES are present on affected folders.