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I have had this issue with the gdoc_field module which does the same thing that this module does, except for Google Docs. The issue we had ultimately ended up being the .htaccess box that often appears on non-production sites to require a name and password before entering the site. Google and Microsoft can only be accessed 100% publicly, so any .htaccess restrictions that require authentication will prevent documents from showing. I hope this helps anyone debugging errors on their Pantheon and Acquia sites for these modules (gdoc_field and md_viewer).