Ranger – “Defense” [Campaign 5, Mission 3]

I approach the second dungeon, which is located in the Dungeon, and I get the message: "The dungeon is empty. Someone has already freed the captive hero." What is this, and how can I deal with it? Version 1.60, Golden Edition. I haven't installed anything on top of it yet; I was planning to play it like this first. Please help!