a bug with incorrect display of the hero's movement range.
It looks like this - unclickable areas appear on the map. Sometimes it only shows the green path, i.e., how far the hero will travel that day, but it's not clickable beyond that. Sometimes it's simply impossible to travel somewhere, even though a visible path exists. Sometimes this is fixed after passing the turn, i.e., on my next turn, and sometimes it isn't. This bug has been observed in earlier versions of HoMM3 HD as well.
I'm attaching a map and a save file; move the heroes in different directions, and you'll understand.
I had the same issue.