I had a similar issue with my iPhone 5. What I did is I got rid of the backup and sync history from the device section in iTunes. To do so, go to the "Deleting a backup in iTunes" section here:
Next, exit from iTunes and start it again. You should now be able to effectuate the sync.
I am having this problem with my iphone 5 as well. Could you please give me instructions on how to delete the back up? When I go into Devices it doesn't appear to give me the option to delete the back-ups.
I plugged my ipad in to see if that worked and I am also getting the "this ipad failed to sync as the sync failed to start" message. I also can't transfer any purchases I've made from my phone/ipad. Soooooo frustrating!!!! Please HELP!!!!