If the requested url is that is failing is "/htdocs/file.swf" and your file is in /htdocs/file.swf then I'm guessing the url that it should be requesting is something like http://domain.com/file.swf.
So if you're using an %ENV varible to come up with the first part of the string "/htdocs/" you're just using the wrong variables. If it's static try just using "file.swf" by itself and see if that works.