On DSM 7, Safe Backup Plus runs a special package-specific user named backuptool. In order to perform backups and restores, this user must be granted permissions to access the appropriate Shared Folders. This does not apply to DSM 6.
Log in to the Synology NAS
Open the Control Panel and go to Shared Folders
Select the Shared Folder containing the data to be backed up and click Edit
In the Permissions tab, select "System internal user" from the dropdown menu on the left
For the user backuptool, click the checkbox in the Custom column