In the Finder, go to menu Go > Go to Folder... and paste the line above then press Return.
I had the same problem. It was an easy fix. Open ITunes, in the upper left select ITunes, then Preferences, next select the Devices tab. You will see your backups in the Device backups window. Select the backup in question and then Delete Backup. Now, you can go back to ITunes and backup the IPhone.
You'll find your backup(s) here: ~/Library/Application Support/MobileSync/Backup ... If you have more than one iDevice, just do a backup in iTunes and then check the timestamps on the folders to find the one most recently modified.
To access it, try to follow this guide about how to extract and recover iPhone backup.
Hope it helps.