When downloading a multi-volume backup file from an external FTP storage in Plesk in Tools & Settings > Backup Manager > backup_timestamp > Download, a file smaller than the real one is downloaded with the following content:
CONFIG_TEXT: Repository error: file 'example.tar' does not exists
When trying to download a part of a multi-volume backup in Plesk in Tools & Settings > Backup Manager > backup_timestamp > [Show Volumes], the following error is shown:
PLESK_ERROR: Error: Invalid type of a backup file.
The following entries are found in the file
CONFIG_TEXT:ERR [panel] Call to a member function getId() on null:
ERR  Call to a member function getId() on null
ERR [panel] Invalid type of a backup file.
When trying to connect to the external FTP storage using an FTP client, the following errors can be shown:
CONFIG_TEXT: Status: Server sent passive reply with unroutable address. Using server address instead.
Error: Connection timed out after 20 seconds of inactivity
Error: Failed to retrieve directory listing
CONFIG_TEXT: Server sent passive reply with unroutable address 192.0.2.2, using host address instead.
Timeout detected. (data connection)
Could not retrieve directory listing
Error listing directory '/FTP'.
This behavior is caused by Plesk bugs PPPM-8828 and PPPM-8830, which are planned to be fixed in one of the next Plesk updates.
Until the bugs are fixed, use the following workaround:
Go to Tools & Settings > Backup Manager > Remote Storage Settings > FTP.
.symbol (period) in the field Directory for backup files storage.
Click OK or Apply to save the changes.