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)"
Solution: Open your SharePoint site in Web browser, Login to Site once. Now, Restart SharePoint designer and try to open your site with SharePoint designer and you should be able to get the site.
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