If you receive:

InfoPath cannot open the following form: <url>
The file is not a valid XML document.
DTD is prohibited.

This problem occurs when the form template that was used to create the form is not available or cannot be downloaded from the site.  The form template was not sent in the e-mail message, or the form template was not published to a shared location.

The resolution is to add the URL for the form template to your trusted sites or local zone and ensure that the person has ‘Contribute’, the minimum permissions required to update InfoPath forms.