Several users have reported that they receive the error “The iPad <iPad name> cannot be synced, an unknown error occurred (1156)” after updating iOS 6. An associated problem is that iPhoto prevents users from syncing photos and videos.
An Apple forums user posted a solution. Following the steps below seems to fix this problem:
1. Sign-out of iTunes Store
2. Disconnect the iPhone and switch off ‘sync over Wi-Fi’
3. Close iTunes
4. Shutdown and cold restart the computer
5. Start iTunes
6. Dock the iPhone and allow the sync to take as much time as needed (it took over 10 minutes for me)
7. Force a new sync and watch as the steps flow through
8. Sign-in to iTunes Store
If the above steps did not solve your issue, try re-installing iTunes.