System.Net.WebException: Unable to connect to remote server System.et.Sockets.SocketException: No connection could be made because the target machine actively refused it 127.0.0.1:80

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For me, this was an easy one, IIS was down.

Other ideas:
1) Check app.exe.config and make sure URL is corect
2) Make sure machine name is correct http://xxxxxxxx or http://localhost