We may reach in a situation where we have configured the plesk to save the backups to a remote FTP repository, but the backup is stored with a size of zero.
This issue may be due to windows firewall blocking the plesk utilities. To solve this issue we can add the plesk utilities to the windows firewall exeption
C:\WINDOWS\system32\inetsrv\w3wp.exe
"%plesk_bin%\plesk_agent_manager.exe"
"%plesk_bin%\pmm-ras.exe"
"%plesk_dir%\bin\pleskbackup.exe"