HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

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I'm more interested in what kind of error causes this. When I play single-player, this happens every turn. Usually, it happens when I try to save or load the game.

I remembered when it happened.
I wanted to save during a turn by pressing the S key, but the game didn't respond. Then, when I tried to save through the main menu and the Save Game button, this window popped up.