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Yeah, had all of my devices forced to sign out, had my account locked and required to do a password change. Been speaking with others on Twitter who had the same thing just happen. I wonder what's going on.
 
I wonder if the password requirements have changed recently [edit: nope, they haven’t changed]. I read on an Apple discussion this happened after a user upgraded iCloud storage, but that seems a bit random.

Macrumors had a story about this in 2018. The users were kicked out and forced to reset the password for security reasons:

 
If you’re all on iOS 14 or have been, it has to do with Apple’s servers. Some other issues include icloud photos and messages in the cloud. In my case, I can’t delete and disable messages in the cloud. I get the download messages loop. Several senior advisors have told me and others that the fix is coming in a future update.
 
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