Most Web browsers (which act as FTP clients) use passive FTP by default because companies prefer it as a safety measure. So switching to passive mode may be necessary with some FTP servers that are behind firewalls that do not allow connections from outside.
To set Backup4all to connect using Passive mode:
On the Destination -> FTP window click on Edit Configuration (or if you're usig a predefined destination, go to File -> Options -> Online destinations -> FTP, select the predefined FTP configuration you want to change and click on Edit)
Check the Use passive mode for transfers (like WWW browsers) option in the Advanced section
After that use the Test button to see if the FTP connection works.
If the Passive mode option is not selected, Backup4all will connect to the FTP in active mode. In active mode, Backup4all will send a connect request from a random port (N) to the FTP server's command port (by default is 21).
Then it will start "listening" to port N+1 and send the a connect command to the FTP server. The server will then connect back to the client's specified data port from its local data port, which is port 20.