You may be able to reset the fog of war for a particular map by deleting the associated shroud.* file in the save folder, or replacing it with one from an earlier save. Make a copy of your latest save before trying that.
Different file extensions are associated with different areas of the game;
.0 / .x0 = Aleroth, Ars Magicana, etc
.1 / .x1 = Verdistis, Council Hall
.2 / .x2 = Dark Forest
.3 / .x3 = Black Ring dungeons
.4 / .x4 = Wastelands
