I have uploaded the new script and updated the code with the link that includes the urlencode() function.
The dwi.php I was running when I sent you the link to the test page was the first version, as the second caused the 500 error. I am now using the latest you sent, but I still get the empty window.
I appreciate very much the help you are giving me, but if it gets too difficult to debug I will just keep it as it is, namely using wp-content for the download data and the faulty button. It's not the end of the world.
I am new to WP and can now see the limits of its much-touted simplicity: as long as you build a lousy blog with twenty-eleven it's fine. Try to customise something and you open up Pandora's box with plenty frustration and time wasted. All this ugly stuff removes most of the pleasure to build a website.