There are certainly multiple reasons for seeing this error. This blog post explains one situation.
The Chilkat.Ftp2.ClearControlChannel method sends a "CCC" command to the FTP server. The purpose of this command is to clear the control channel such that commands/responses sent over the control channel are unencrypted, while data transfers (uploads, downloads, and directory listings) remain encrypted (SSL/TLS). The reason for needing to clear the control channel is to allow a NAT router to translate from internal IP addresses to external IP’s so that a data connection can be established in non-Passive (i.e. Active, or "port") mode.
Some FTP servers do not support the CCC command. Unfortunately, it is not programmatically possible to know whether a server supports it without simply trying the command and testing for an error response. (It is not a "capability" reported by FTP servers.)
This is an explanation of how you might get the 425 FTP error when uploading a file, downloading a file, or doing a directory listing (LIST / MLSD):
If you’re using non-Passive mode (i.e. "port" or "Active" mode), then the data connection is established by the client sending a PORT command to the FTP server with the IP address and port # of the data connection endpoint. The server initiates a connect request to this endpoint, and the client listens. However, if the CCC command is not supported, the control-channel is still encrypted. If the IP address in the PORT command is an internal IP (not an external one), it probably begins with "192.168.". Normally, your router would translate the IP from an internal address to an external address. However, it cannot because the control channel is encrypted. The server receives a PORT command with an IP address of "192.168…." and tries to connect to it — but fails and thus you receive the "425 Can’t open data connection" error (on the control channel).
