Backup to remote FTP server fails: Curl error: Unable to resume an interrupted upload: (55) Failed sending data to the peer

Symptoms

• A Plesk Backup to remote FTP server fails with one of the following errors:

  Error: server "server"
  Curl error: Unable to resume an interrupted upload: (55) Failed sending data to the peer: Last FTP request: APPE backup_1801300838.zip: Last FTP response: 150 Opening BINARY mode data connection for backup_1801300838.zip: Connection to the FTP server has lost

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  Unable to create remote backup. Error: Unable to export backup: Curl error: Unable to resume an interrupted upload: (55) Failed sending data to the peer: Last FTP request: APPE backup_2008150549.tar: Last FTP response: 150 Accepted data connection: Connection to the FTP server has lost

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  Unable to create the remote backup: Curl error: Unable to resume an interrupted upload: (55) Failed sending data to the peer: Last FTP request: APPE backup_1801300838.zip64: Last FTP response: 125 Data connection already open; Transfer starting: Connection to the FTP server has lost

Cause

The FTP server or FTP user exceeded quotas or limits, e.g hard disk quota, in the middle of the backup process.

Resolution

Check and adjust the limits and quotas for the FTP server and user.

Note: if this limit is a FTP timeout, it is possible to avoid this issue by enabling multivolume backup in  Tools & Settings > Backup Manager > Schedule > Create a multivolume backup, volume size (MB).