"iCloud... I'm Already Signed In!" Stop with the "This iMac can't connect to iCloud... Open iCloud preferences to fix this problem."
I don't know what the problem is but these windows keep popping up.... after login and while logged in. Clicking on "Later" or opening "iCloud Preferences..." makes no difference.
But I'm already signed in! Another Post-El Capitan upgrade glitch? They're minor annoyances but piling up to become a major aggravation.
iMac (Retina 5K, 27-inch, Late 2015), OS X El Capitan (10.11.1), 24 GB Memory