the FTP protocol is quite unique, because it opens a data connection for each transfer / listing, etc. (The port 21/990 is only used for the control connection).
So my guess is that most probably this is some firewall configuration error or some network issue. You need to open a range of ports for the data connection to go through the firewall. The error that you get simply says that the data connection could not be opened.
Passive mode is more firewall friendly, so I'd suggest you to start with Passive mode (default in Rebex FTP) like this:
Ftp ftp = new Ftp();
// create a log of communication
ftp.LogWriter = new Rebex.FileLogWriter("log.txt", Rebex.LogLevel.Debug);
For more information about ports used for control / data connection in FTP see
The exact same error that you get is discussed at
Let me know whether you were able to solve the issue.