After rebooting my Mac recently I have been getting an iCloud login prompt pop up. It would not accept my credentials so I just had to cancel it.
All the iCloud functionality still worked. Eventually I got fed up with it and had a quick search on the net to see if any one else had the issue and if there was a fix.
Well it turns out quite a number of people had the issue but the fix was very simple. Just go into your iCloud settings, logout then login back in. Problem solved.