Note: This article has the reference to the issue with the fix available:
"Extensions were not included in Plesk backups if there were one or more symlinks in the folder containing the extension’s files."
- Plesk Onyx 17.5.3 23 March 2017
Please consider updating your server:
Unable to create full server backup. The following error is shown:
backup_restore_helper utility returned incorrect result. Extension with ID 13 cannot be backuped. Please, contact your service provider
Bug #PPPM-5480 . It was fixed in Plesk Onyx 17.5
Upgrade to the latest version using Plesk installer.