You received that error because some files of the game were replaced by new ones after you installed the patch. Since your saves are using the old files, you will not be able to access them because those files are no longer available. In order to fix the problem, you will need to remove the patch (Control Panel > Uninstall a program). If this doesn't help, remove completely the game from your PC and then install it again. After that, you should be able to access your saved data.