It looks like it can't find a folder or file.
As it's 'copying' you should have an update.log file
this will list all the files folders it has copied across.
Theory is if you look at the last one successfully copied the next one (folder or file) is the one it is having problems with.
BTW are you syncing between Drives on same PC, two PC's on a wired lan, USB stick and PC or two PC's seperated by wireless?