Try this: open backup - apps+settings turn off settings backup and apps backup...Then go to manage backups and delete everything there. Go back to backup and turn on settings backup only, click back up now, it should work. Next step is to turn on settings backup and app backup together and click backup now.
Note: Make sure that you are on steady and fast internet connection, close all apps running, music player, photos, third party apps like vlc and so on. Sometimes backup fails at 99% on Wi-Fi, or fails because some app is trying to be included in backup and fails miserably , and finally if you deployed some apps it can fail also.