iCloud Backup: "The last backup could not be completed" I don't exactly remember when this started but my iPad running iOS 5 started not doing backups to iCloud. When I press the "Back Up Now" button it starts working for a few seconds but then stops and shows the message from the title beneath the button.

I already tried reseting, restoring, deleting my iCloud account from the settings, using another iCloud account and now I'm pretty devastated.
Okay, I figured it out now.

After getting to replace my iPad due to a hardware defect I installed the iOS beta I have been using before and forgot to register the new device as a developer device as well.
Stupid mistake, took me weeks to identify the reason for this behaviour.