Q: iCloud automatic backup

My sister has my old iPhone 5.  Recently, her phone will not automatically back up to iCloud.  She gets an error message ("unable to validate").  If she tries to back up manually, she cannot.  If she reboots, she can back up manually, but the same thing happens overnight when she is plugged in.  I have tried a soft reset with her, logged her out of iCloud and back in, checked all of her settings, deleted one stray old backup.  The only thing I haven't tried is to remove all apps from backup and try again, but I wouldn't think an app would interfere with iCloud verification for backup.  Anyone have a suggestion (besides a total restore).