Google+ Auto Backup failed When looking at my photos in the Google+ app, at the bottom is a footer that says "Backup failed" with a retry button. The backup still fails when I click Retry.

Is there a way to find out why Auto Backup is failing so I can correct the problem?
Try deleting Google+ data/cache, then start Google+ app. It will log you in and then you can choose Auto Backup setting. You can then go to Sync settings, untick "Sync Google+ Auto Backup" and then tick it again. Seems to work for me. You can also go to the Google+ app settings and get it to upload all your photos/videos to start the sync.