My iPhone 4s was connected to a power source and has WiFi connection. The iCloud have enough free space. When I manually backed up to iCloud, I got the error message after several minutes: “the last backup could not be completed.” I had tried restored my iPhone, re-installed iOS 5.1 and deleted the old backup in iCloud, but I still have the same issue. Same problem with my iPad 2 and iOS 5.0. Now I am not able to have done a backup.
How to fix “The last backup could not be completed” issue
* Plug your iPhone/iPad to your Mac/Windows using USB cable.
* Open iTunes software and choose “backup to this computer” .
* Then you need to do a sysncing.
* Go to iCloud and delete all previous backups.
* In iTunes, Turn on “backup to iCloud” again.
* On your iOS device, now you are able to backup to iCloud.