A few turns in the 1918 scenario there's an error where the German player must play a card. But none of the German cards are playable (purple). As a result the turn is unable to progress thus rendering the game unplayable. The error occurs both when the German side is controlled by the player or the computer.
There's a similar post on Steam so it's definitely a bug that must be fixed.
Yes, we shall investigate that at once. Do you remember the card name, Or can send a save just before this to our support: