Periodically attempt to open the backup via Plesk located on the FTP storage due to the following error:
CONFIG_TEXT: Error: The backup you are trying to restore was created in an unsupported version of Plesk and can not be recovered. Use the Plesk Migrator extension to transfer your data from legacy Plesk versions.
When the issue occurs, the following error appears in the "/var/log/plesk/PMM/migration.log" file:
[2018-12-25 21:25:19.508|16633] INFO: Unable to transfer file in active mode. Switch to passive mode
[2018-12-25 21:25:19.508|16633] INFO: Curl error: (19) FTP: couldn't retrieve (RETR failed) the specified file: Last FTP request: RETR backup_1812211336_1812251848.tar: Last FTP response: 425 Unable to build data connection: Operation already in progress
The root cause is unclear.
Enable Use Passive Mode option at Tools & Settings > Backup Manager > Remote Storage Settings.