Have you checked that it is not permission issue or firewall issue?
Try creating one simple html file with link to your .pptx document which you are trying to open from sharepoint webpart, if you are able to open document then probably it has some permission issues within sharepoint site.
It might sound little dumb, but hope you have check the path which is generated by your sharepoint webpart is actually pointing to .pptx file which you are trying to open! If that is incorrect then it has something to do with webpart coding.