The following message appears during configuring FTP repository or notifications with the same text are sent during the scheduled backup:
PLESK_WARN: Warning: Unable to resume an upload of the file to the storage: the content of the retrieved file does not match the content that was previously sent.
FTP server does not allow resuming uploads in case when the upload was interrupted (e.g. if there are some network issues).
Some FTP servers do not have this option enabled by default.
Allow FTP server to resume uploads:
For Linux with ProFTPd FTP server, check the following article AllowStoreRestart
For Windows Server, check the following article FTP File Handling <fileHandling>