If you have enabled Apple's two-factor authentication and you suddenly find your laptop fan is rather insistent or your trickle rates on the desktop have dropped or your iPhone is red hot and running out of battery power then you might find that it's possible to regain control of your devices by reverting to passwords. On Windows the service is called "Apple Security Manager" (secd.exe) and it runs flat out, indefinitely (or as long as my patience lasted). Updating iCloud to the current Windows version makes no difference (6.1.1).

Though Apple maintains its usual infuriating inscrutability about these sorts of bugs, it will no doubt issue a fix in due course, if you can stand the heat/noise/annoyance. In the meantime old-fashioned passwords seem to work fine. Signing out of iCloud usually did the trick after reversion, though a few iClouds had to be uninstalled/reinstalled to kill the service off.