Three paths.

There are two main possibilities: either some mod (which can include certain maps) was installed/uninstalled, causing the save files to become corrupted. Or the save file was corrupted due to its large size – this usually happens in the later stages of the game and on large maps with a very high density of objects. The latter is unlikely in this case due to the map size, although it is theoretically possible.
Otherwise, I don't know any other reasons.