Confirming iCloud password to use Backup?

Dear Apple Support Communitiy,

since some weeks, my iPhone 4S (iOS 6.1.3) and my iPad 3 (iOS 6.1.3) are asking me to confirm, while charging, my iCloud password in order to use the iCloud Backup. This is very annoying. I use iCloud right from the beginning, but I never had such a strange issue.

P.S.: I did not change my password since some months and I have 2,8 GB left on my iCloud account. All the other iCloud feature are working perfectly.

Any help is welcome!