Which means that the page redirected you to a non-existing page. Either the server-sided script is broken, or you haven't provided the same information that you normally would if you submitted the form in your browser.

Or there's something different with cookie-handling or so.

The easiest way to find out is to do the same request once in a browser, and once in a script, and record the HTTP headers (both request and response) with a tool like wireshark, and then compare them.