not entire sure what causes it specifically, but deleting many cities may crash the game when saving.
The game will run just fine up until saving and it won't always crash.
After crashing, there will be a _temp.sve that is unreadable, but not always.
Sometimes the game will crash right after completing the save, which can be loaded and played on just fine.
It is also possible to crash after deleting cities and then loading a previously saved game.
I think that I have found and fixed this - thank you for the report!