Steps to Reproduce

1. Create an Application Link between Confluence and JIRA
2. Set the Display URL to a non-existent address, or to a proxy address that is reachable from the client's browser, but not accessible from the Confluence Server itself (the whole point of having an Application URL / Display URL separation)
3. Create a page and add a JIRA issue into the page
4. Export the page to PDF

You will notice Errors such as these:

2014-03-22 17:57:51,167 WARN [http-8542-1] [confluence.extra.flyingpdf.ConfluenceExportUserAgent] getImageResource Could not retrieve image resource http://192.168.2.3:8614/jira614/images/icons/statuses/open.png during Confluence export
 -- url: /c542/spaces/flyingpdf/pdfpageexport.action | page: 1933317 | userName: admin | referer: http://localhost:8542/c542/display/GIE/sa+dasd+as+d+assad | action: pdfpageexport

It seems that the getImageResource will attempt to obtain /images/icons/statuses/open.png via the Display URL, and not the Application URL. The Confluence Server is unable to reach the Display URL, hence, the error

Workaround

Set the Display URL to a URL that the Confluence Server is able to reach