Let’s say you have upgraded your iPhone 5/4/3GS or iPad to iOS 6.0 firmware (or iOS 6.0.1) and you have also updated iTunes to its latest version (11.0.1), but while syncing all of your music, apps, contacts, the media player goes bonkers, repeatedly throwing out 1156 error. What can you do now?
Do not confuse this error with the 11156 problem, which occurs when you are trying to do something that is not allowed by the iTunes Store in your country. If iTunes player pops up a window with “The iPad / iPhone <name of the device> cannot be synced. An unknown error occurred (1156)” error message, then we have a solution to this problem.
The fix is very simple: sign out of iTunes, disconnect the device, restart your computer and finally log back in to iTunes using your Apple account. This sounds like a generic solution, but it works! As posted in the Apple forum, here are the complete steps you must follow: