If you have deleted the backup set in Pro Backup without deleting the files first, you may have files on the account that cannot be deleted from the storage space on Storegate. This is because by default you should not be able to delete a backup.
To get around this, you can log in to the account on the old website: https://secure.storegate.com/. Then add &statusCode=507 to the URL when you are backing up the website.
Example
So: log in via the website and press backup. Add &statusCode=507 to the URL when you are in the root of the license you want to delete archives from. Select the archives (folders) and choose manage > delete.
Sometimes the URL is wrong. Then you can switch to my sync files and then back to the backup files. The link must not end in =