I've noticed...... I'm not sure how relative this is, but I've noticed with custom_location.txt maps.
hmmm.... Ok suppose you have custom_location.txt map that in the campaign there's a settlement located in that map grid. When creating a game on the same grid and installing a fortification of some type. The game will CTD. This seems to be because on a custom_location.txt map at a grid that has a settlement........ the computor will generate a building or town. It seems that is what happens in these locations is...... the computor is generating a structure at the same time the host is attempting to generate a structure. I get the impression that the two things conflict for space on the map, causing the CTD.

I'd have to say, that it's impossible to place a new customized map grid over an exsisting settlement grid without 1st removing the previous (original) settlement data completely from the game.

Just a thought.

Yes, I realize this is a extremely old post. But the info may be of help to future modders.

Thompson45cal.