Is it possible to restore backup protected with Plesk's encryption key (Tools & Settings > Backup Manager > Settings > Backup security settings) on another Plesk server?
According to this documentation page the key is unique to each Plesk installation. Encrypting backups with Plesk keys is convenient but has certain limitations. When users restore backups created in other Plesk installations, the data is restored, but the original user passwords are lost, and new passwords are generated instead. In this case, the users will receive password change notifications by email. If it is required to restore a backup on another Plesk installation and want to avoid changing user passwords, use the Specified password option in Tools & Settings > Backup manager > Settings.
It means that encrypting protects not the files itself, but user's access data, so it is possible to restore files, configurations and databases on another Plesk server but user's passwords will be changed.