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Resetting the Lockdown folder can resolve issues with starting the backup session or saving the backup. If you can back up in a new user, do not reset the Lockdown folder. Issues with the Lockdown folder occur across all users; they are not user-specific. Follow these steps to reset the Lockdown folder:

Mac OS X

1. Disconnect all iOS devices from your computer and quit iTunes.
2. From the Finder, choose Go > Go to folder.
3. Type the following and then press Return:
   /var/db/lockdown
4. Choose View > as Icons.
5. The Finder window should display one or more files with long alphanumeric file names.
6. In the Finder, choose Edit > Select all. Then, choose File > Move to Trash. Enter an administrator password if prompted.
   Note: Do not delete the Lockdown folder itself. Delete only the files contained inside the Lockdown folder.
7. Reconnect your iOS device and open iTunes.