Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Windows SharePoint Services-compatible HTML editor such as Microsoft Office SharePoint Designer. If the problem persists, contact your Web server administrator.
This is a XSLT error.
The question is, is it a sintax error or is the query unable to retrieve the data?
- Enable custom errors and open the page with SharePoint designer to look for XSLT errors
- Add the existing URL to alternate access mappings in central admin (edit public URL) if you're using a secondary/external URL