Problem: Tried opening a SharePoint 2013 site in SharePoint Designer 2013 and got this error message “The server may not have Microsoft SharePoint foundation installed, your machine’s IP address may not have access to the server, or you may have specified an invalid proxy server (HTTP Error 403)”
Root cause is: The particular SharePoint site is published with TMG! SharePoint Designer 2013 couldn’t pass the credentials to TMG and resulted in HTTP 403 Error.
Solution: Open your SharePoint site in a Web browser (preferably, Internet Explorer), Login to the Site once (So that it creates an authentication cookie!). Now, Restart SharePoint designer and try to open your site with SharePoint designer, and you should be able to get the site.