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

I think the problem is that the heroes are not installed, but only copied – and the mod is installed.

On 2.98, this didn't affect anything, but on 3.45, it probably did.

How do installed heroes differ from copied ones? Nothing. The problem is somewhere on the surface. And in such cases, they don't guess, but simply ask their question and offer to share their screen.