My statement is, "Keep it as simple as possible." So when you face issues with testing the report server, start by taking Application Express out of scope.
One way to make sure that your print server is up, running, and configured properly is to set up a static HTML form that simulates what Application Express is doing internally, that is, posting some XML data along with an XSL-FO stylesheet to a print server via HTTP and receiving back a PDF document as the response. If this works properly, you can at least be sure that your print server is working.
The example file, How To Debug APEX and PDF Printing Integration Issues: BI Publisher / Apache FOP (Doc ID 454701.1), can be downloaded from ...