Heroes 5, Tribes of the East

The fifth mission for the Orcs, the entire map is cleared, artifacts are collected, and all enemies, except for the last castle, have been defeated.
When attempting to attack this very castle, nothing happens; the hero moves right up to it, and that's it. If you skip a turn in this state, the game simply crashes with an error.

I haven't installed any patches, and I've never had this problem before. Can you suggest how to deal with this issue?
Ideally, I'd like to avoid having to replay the mission.