I can't ftp my snapshot file. Looking at the logs, it fails

I can't ftp my snapshot file. Looking at the logs, it fails with the following error:

2014-05-21 12:17:38: Sending Archive: snapshot-1400659778-140521-111731-31a4fd98.zip 16.88M
2014-05-21 12:17:38: Destination: ftp: Microserver
2014-05-21 12:17:39: Warning: errno:2 ftp_put(): Illegal PORT range rejected. /home/sites/silcoatesdiploma.co.uk/public_html/wp-content/plugins/snapshot/lib/destinations/ftp/index.php on line 665
2014-05-21 12:17:39: Using FTP connection
2014-05-21 12:17:39: Connecting to host: xxxxxxxxx (blanked out for security reasons) Port: 21 Timeout: 120
2014-05-21 12:17:39: Passive mode off.
2014-05-21 12:17:39: Timeout set to 120
2014-05-21 12:17:39: Login success.
2014-05-21 12:17:39: Home/Root: /
2014-05-21 12:17:39: Changing Directory: /FTP
2014-05-21 12:17:39: Current Directory: /FTP
2014-05-21 12:17:39: ftp_put failed: snapshot-1400659778-140521-111731-31a4fd98.zip
2014-05-21 12:17:39: ERROR: Undefined index: verification-bing

The ftp test is successful and I can also connect manually and create files and folders