There are several possible explanations for this problem, but the FTP LIST command usually fails because the passive IP address being given out is incorrect, or the passive port range hasn't been forwarded from your router to the machine running Cerberus FTP Server.
Your network may be running a firewall, or your computer may be behind a router. See the section on Make your FTP server accessible from the Internet for more information.
If you are using a router with a cable or DSL to connect your computer to the Internet, you may need to enable port forwarding. Consult your router documentation on how to enable port forwarding for FTP.
Finally, if you can, instruct your clients to connect using passive FTP mode (initiated by the client using the PASV command). Active FTP (initiated by the client with the PORT command), requires the client's network to be configured to allow incoming data connections. Active FTP will rarely work outside of local networks.