Exporting a backup to a file over the CLI does nothing, just waiting:
# /usr/local/psa/admin/bin/plesk_agent_manager export-dump-as-file --dump-file-name "/var/lib/psa/dumps/domains/example.com/test_poi123.com_info_1511061556.xml" -v --output-file "/home/testback.tar"
Wrong utility is used for the exporting.
The following command should be used instead:
# ./fullbackup.php -e /var/lib/psa/dumps/backup_info.xml <output_file>
For more detailed description of the utility, please, refer to KB article.