your file is corrupt, and it wants you to restore the auto backup, or your own latest backup.
in the save folder are .old files, remove*** the offending file, rename the .old one by removing the .old part of the name, and launch the game again.
game folder is in your profile under appdata somewhere for windows, under the iron gate name.

*** if you do not know what you are doing, move it to another folder so you still have it if you screw with the wrong file. It should be obvious, but I say that and people still hard delete the wrong file and lose their world.... so, don't do that until you understand how their filesystem works.