This issue was brought up many times in different forums, with no real solution (try searching the Web, there are results starting from 2009).
After getting a laptop with Windows 7, I cannot access non-public FTP sites any more. Looks like it is a common problem, and I'm still convinced it is because of Win7. The Windows Firewall is disabled and I've tried many different connection points so this is not an issue of a router either. For the same connection points, a laptop with Windows Vista connects to the FTP just fine. A friend with a Win7 machine has also confirmed the same behaviour I experience: either the FTP client won't connect at all or after connecting, won't display the file list or allow any download/upload.
FTP clients checked: command line, Windows Explorer, FileZilla, Ipswitch WS_FTP, Cyberduck. Of all these, only Cyberduck shows successfully the file structure, however any transfers fail one way or another.
I'm really lost and need help, since I rely on those private FTP sites and need to access them from the Win7 machine...