I've found a way to fix that !!
Uninstall the game without removing your saves.
The bug came with the last update of the game, so when you will start it again, don't do the update. You won't be able to play online as long as you don't update it, but you will be able to finish the game !! Then, just update it if you wish to play online or don't if you don't care. Et voilà !