Plesk backup to remote FTP finishes with warning: Unable to upload the backup to the external storage: Unable to resume an interrupted upload.

Symptoms

Backup to remote ProFTPd FTP server finishes with a warning:

Unable to upload the backup to the external storage: Unable to resume an interrupted upload. It seems like FTP server deleted the file, when the connection was lost. Try to repeat or perform upload manually: The connectivity problem: (55) Failed sending data to the peer: Last FTP request: STOR backup_1905231015.tar: Last FTP response: 150 Opening BINARY mode data connection for backup_1905231015.tar

Cause

FTP server misconfiguration: restarting interrupted uploads is not allowed.

Resolution

1. Connect to the remote FTP server via SSH.

2. Add the following lines to the /etc/proftpd.conf configuration file, or edit them as follows, if they are already present:

   DeleteAbortedStores Off
   AllowStoreRestart On