But if i do it manually it fails. Why?
Try managing your backups. You can try deleting everything then trying to backup, or do a soft reset, or disable app/settings backup.
Is your email asking to be verified?
Hard Reset may fix it for the first day after, but then the error represent itself, so it's not a definitive solution for now.Mine used to do this - it used to get to 96%, wait for a while and then fail with an error message.
A hard reset resolved it.