BitBucket credentials popup broken in Visual Studio's Team Explorer

I’m using Visual Studio 2013 Update 4 on a Win7 64 machine.

I’m following this tutorial to play around with BitBucket. Everything works fine up to the point where Visual Studio throws the authentication message box. This is what it should look like:

expected popup

The problem is that the dialog I get shown doesn’t contain the middle part. There’s the “Remember..” check box, and the buttons, but nothing to actually enter anything. OK ist deactivated and when I click Cancel I get an unathorized error shown in Team Explorer.

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