Error 510 when generating from address with URL variables

I'm using the PHP api and having trouble converting any address that contains more than a single URL variable. Since this is a public forum I can't post the actual URL because the page may contain client data. The structure is pretty basic though. If I pass an address like:

http://demo.software.com/output.php?id=555

I get a PDF and it works great. But if I add a further URL variable like:

http://demo.software.com/output.php?id=555&key=a7cd3372a

Then I get the 510 error:

[510] Timed out. Can't load the specified URL. List of the resources that can't be loaded: http://demo.software.com/output.php?id=555&key=a7cd3372a

However, if I test the address that it claims can't be loaded by browsing to it the page loads fine. And, if I use the converter on the main page of the pdfcrowd.com site or use curl from the command line with that address I get the PDF I'm expecting.

I watched my server logs to see what was going on and when the additional URl variables are passed in the server doesn't show any activity for quite a few seconds and then returns just a GET to http://demo.software.com/output.php?id=555&key=a7cd3372a but doesn't load any of the additional files that are references on that page. Without the second URL variable the full list of files loads almost immediately.

Is there something I should be doing differently? I'd like to purchase a license, but it's essential that I'm able to pass url variables.

Thanks for your help.

There may be several reasons causing the 510 error. Please, send us the actual URL to info@pdfcrowd.com and we will look into it.