Paizo - What you have here is a software problem: You seriously need to rethink the way download is implemented if people are to start being able to download things and the queue can be cleared.
What happens is the user session is timing out BEFORE your site is able to produce the watermarked PDF. Apparently this means that, whatever work had already been done in producing the PDF is lost, as when you re-login and re-click you seem to go back to the PDF-produce queue.
This means that your server is being hammered... but no productive work is being done and no one is leaving the queue.