Steps to reproduce:
(1) enable "dialog_tool[27]=,i" in menuconf.tab (it is not essential to use the "i" key, but I find this a good key for this tool;
(2) start a new game and use this tool by pressing the "i" key (or whatever key was assigned);
(3) press "simuconf.tab" to reset all the default values to the simuconf.tab defaults;
(4) save the game;
(5) reload the game - it will fail.
Cause:
when the "simuconf.tab" button is pressed. all the values in einstellulnen_t are reset - including things such as the number of cities and the size of the map, which are essential for correct saving/loading. This should only allow the changing of values that can safely be changed without breaking things.
Edit: Similar problems are caused by editing the climate settings (water level, for instance), although this does not necessarily break the savegame - but it does flood areas that were not previously flooded when the game is re-loaded.
Imho the real number of cities should be taken from the map itself, same for the size. Aparently this is a design error, that should be fixed sooner.
Ahh, yes, you're right: that would be a better solution.