Did you put the word "min" after your script or style URL?
Something like
<link rel="stylesheet" href="mycss/screen.css" type="text/css" class="min" />
I know this is not exactly XHTML correct, but it’s a quick fix to allow you to exclude stylesheets and javascript from minification. A blank file usually means that no good javascript or stylesheet includes were found.
If that’s not the problem, are you having any PHP errors or MODx errors logged as a result? Also, this script uses fopen and file_get_contents, so check if all security permissions for those are available.