Depends what size the map_heights is. Despite way its normally refered to its not actually a ratio problem but relates more to absolute size and shape of sea's - but unfortunately not something I've ever pinned down to a quantifiable size / shape!

Guessing visually if that heights is over 700+ high you could have a problem especially with the wide sea area to the west of italy.

To test if it is that or not simply make back up copy of your map_heights then alter in game one by putting large RGB1,1,1 islands in the middle of the widest sea areas - or if you don't have ports on map_regions / descr_strat you can simply try launching with an all land (1,1,1 plus) map heights. You don't need to alter map_regions or anything else to test.

BTW you can also get similar crash if you have too much impassable land in ground_types, and possibly for very large region sizes (though I think that one does get past generating map.rwm and CTD's at end of load bar - not sure though)