Thank you for your reply. I have triple-checked the location of both the .aspx file and the .cs file, and they are both in the right place.
Is the URL to the .aspx file relative to the XML file, relative to the .swf, or relative to the application file?
The strange thing is even if I type the URL to the .aspx file directly into my browser, it still gives me a 404. But I'm looking at the directory on my server and the file is there.