This error message will appear when data has been stopped from being either uploaded or downloaded. The most common reason for the problem is that a Firewall has barred the data flow between your machine and the web server.
Firewalls do this because they are designed to guard a network like an office, government, university etc, from being broken into electronically. They achieve this by stopping certain sorts of data transfer taking place. Unfortunately, a piece of FTP software will confuse the firewall so it will stop you accessing the web server.
The easiest remedy for this is to put the FTP software into "Passive" mode. This will usually enable you to FTP through the firewall. This setting will usually be found in either the 'Advanced' settings or the 'Firewall' settings. If this fails then you will have to speak to the system administrator about the firewall settings.
The only other option is to upload from home where you will not get firewall problems.