Some FTP servers require the connection to be established in a passive mode (i.e. your computer establishes the connection so the flow of data is set up and initiated by you).
Most server companies have started using Passive FTP mode for the ir clients' convenience. Unlike Active FTP, Passive FTP mode provides does not interfere with the client's firewall, as it does not involve the server accessing random ports on the client side which is generally interpreted as an attack. Backup4all is also using Passive FTP by default.
To check the connection type which is currently in use, open the FTP Configuration window, either by accessing File->Options->Online connections->FTP, selecting a predefined connection and clicking on Edit, or from Properties->Destination->Edit configuration, for Jobs that use a custom FTP configuration.
If the Use passive mode for transfers checkbox is not checked, then the connection is in Active mode. You should always test the connection after making a configuration change.