Software update always tells me Xcode 6.2 is available. Other updates install when requested, but when I click the XCode update button it asks me for Apple ID and password. Other items update without asking.
When I enter those, I never get an error message, and the button dims. But nothing else ever happens. Other things will show a progress bar and change the button text to say what is happening.
I accepted the workaround even though it's not exactly an answer to the question as asked. But it's worth noting here that whatever caused it has been magically corrected.