I have the same problem, but I believe I have found a workaround.
I went to the folder: "C: \ Program Files (x86) \ Microsoft Visual Studio 12.0 \ Common7 \ IDE \ Extensions \ Microsoft \ Windows Azure Tools \ Common \ KeychainDlls"
I renamed Microsoft.VisualStudio.Services.Client.dll to something else (e.g. __Microsoft.VisualStudio.Services.Client.dll)
Then I restarted VS2013 and I was able to log in.
Then finally I renamed it.
I hope this helps
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