I can’t take credit for this solution, it came from BrianBatchelder on the apple forum.
I had to go to “XCode” -> “Preferences” -> “Accounts”, where it turned out my Apple ID had timed-out. After logging-in there, and then restarting Xcode, my profile was automatically “repaired”, and all is now well.
Re-starting Xcode is important there, it doesn’t work without it.