Forgive me if this is the equivalent of asking "is it plugged in?" but are you reloading the file downloads page between attempts to download? The download URL is generated anew for each download of the file, so if you download the file and then click on the link again without reloading, you'll get that error message. It's dynamically generated to avoid people being able to leech your file products. But if you're getting that message and people are still getting the download popup, then that doesn't really seem like the problem...