The '7.xml' file (and all other numerical XML files in the 'uploads/juicebox/' directory) store settings for each gallery but they are not the actual XML files used by Juicebox when the galleries are displayed.
The XML files used by Juicebox when the galleries are displayed are generated dynamically by the 'config.php' file (with a 'gallery_id' query string so that Juicebox knows which numerical XML file to take the settings from) and it is because the plugin cannot access PHP files directly that the "Config XML file not found." message is displayed.
Try temporarily deactivating all plugins (other than WP-Juicebox) and reactivate them one-by-one (checking the functionality of your Juicebox galleries after each one) to see if you can find a conflicting plugin.
The problem seems to be one of security so if you have any security-related plugins, I would try disabling them first.