Cannot sign into Mac App Store in Mountain Lion

When I initiate an update download through Software Update in Mac OS X 10.8.5, or when I try to log into the Mac App Store, after entering my account information the sign-in dialog box just sits there and the action icon spins, but nothing happens ... even after a very long time. Bug in Mac OS? Or something peculiar about account setup and accessing? This is an amazingly persistent problem and is reventing software updates. Any fixes?