I can upload or download files using FileZilla (with FTP), but my application fails w/ the data connection. I can connect to the FTP server just fine, but when I try to upload, or download, or fetch a directory listing, the connection is rejected or it hangs until it times out.
This is a common question.
There can only be 2 reasons why FileZilla works, and your application gets blocked.
1) Windows Firewall can be configured with exceptions -- which are "known" applications that are allowed to bypass the firewall. When you install FileZilla, it adds the exception to Windows Firewall to prevent the firewall from blocking the data connections. You must do the same thing for your application.
2) Whatever you're doing with FileZilla, you're NOT doing the same thing in your application. Check ALL the connection/data transfer settings, especially with Active/Passive mode, explicit/implicit SSL/TLS, and numerous other settings. (Also, make sure you're actually talking to the same server, and make sure FileZilla's not using SSH/SFTP, but your app is using FTP.)
See http://cknotes.com/determining-ftp2-connection-settings/ for details. All of the options Chilkat provides should correspond to something in FileZilla that can be set. I'm not aware of any FileZilla feature regarding FTP data connections (in terms of getting through firewalls) that Chilkat does not also provide.