Family Tree Maker 2012 Error while Synchronizing

I launched Family Tree Maker 2012 and sync it successfully. And then I added some materials to Ancestry but without syncing. And when I try to sync it again, I got an error "Error occurred while synchronizing this tree. You can try again later but if this message returns you may need to unlink your trees and upload a new linked tree from FTM." Can someone tell me what the problem is all about?
