I am behind a corporate proxy server which requires credentials. I have been trying to connect to a TFS server (on tfspreview.com) with MS Visual Studio Pro 2012
I faced the same error with my Visual Studio Team Services account (formerly Visual Studio Online, Team Foundation Service).
I simply entered the credentials using the VS 2013 "Connect to Team Foundation Server" Window, and then connected it to the Visual Studio Team Services Team Project. It worked this way.