Plesk Server Error 500 when downloading cloud back-up to server

The following describes an issue I encountered on our Ubuntu installation with Plesk 17.8.

Plesk 17.8 backup has the option to store locally, via FTP or in the cloud. When trying to move a backup from the cloud to the server using the Copy to Server Storage (Home -> Tools & Settings -> Backup Manager and then choose a cloud backup instance), it'll download it by default to the root folder. This filled up our root and caused the following error when trying to use Plesk:

Accessing Drupal 8 Installed in Subdirectory

Goal: Leave access open to other directories in web root. Don't hack the core for easy site maintenance and to prevent opening potential security holes.