So now I'm running long tests to find the functions which might cause the crashes.

In general, it is pretty safe to make changes early in-game on the first turn (except the impassable-to-passable and water to lands and vice versa).

It was not changed with the mod updates and if you start to get crashes after some mod update it does not related to this update in 99% of the cases it just your game went to the stage where the significant map updates are not safe anymore. The game was not designed to use these functions in mid-game after the players and AI appears on the map and this is the main reason for the crashes. These functions are used by the game on the map generation stage before the game starts. Mod uses game testing and map generation functions to make map changes. The only way is to load the save game made before the problematic change was done but the crash may happen 400+ turns later and you literally can lose all progress. The problem with crashes is that they happen on some turn later in-game and it is not possible to continue the game because it will repeat on that turn no matter what you do.

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