Thank goodness for this post, because we had the exact same thing happen to us with Enterprise Desktop R24. Spent hours and hours trying all sorts of uninstall/reinstalls, running the Tool Hub and File Doctor, on the phone with support, etc, and nothing was working. Found this post and symptoms were an exact match, and sure enough an install of the client on a non 2012R2 workstation got our access back. Pretty crazy that they are able to just drop a forced automatic update that removes support for an operating system with zero form of notification.