I'm running 8.4.1 on an iPhone 5S. Every few minutes, I get a pop-up screen to sign in to iCloud. I do so, I know I'm using the correct password, prompt goes away, then pops back up. I checked settings, and it looks at though iCloud is running OK, fully backed up just a couple of days ago. I've tried Cancel as alternative, but the prompt still comes up. Shutting down and restarting doesn't fix it. Nor does doing a full restart. Nor does logging in to iCloud from my MacBook. Nor does going to a different location with a different Wi-Fi server. Is there a fix, perhaps?