The error should be just on Game.agf, if you can open it on notepad++ or other text editor my guess is it will not be complete. Your scripts you can probably update the code fine by copying and pasting.
You can also try to just use the backup of game.agf with the other files, but it's hard to say. Dialogs are stored in game.agf, so loading the corrupted one in notepad++ may be helpful to find them and be able to restore what has changed in the new one.
There are softwares that can compare text files, like meld, that can be helpful in this situation - load both Game.agf the one that is corrupted and the one from the backup and see what has changed.
I don't particularly remember other XML files in the game project so this is why I guessed the problem is in game.agf from the stack referring a XML reading library.