The Bastioneers mod. 
Ive been playing the mod a fair bit and it has successfully auto-saved before, so I’ll check what I’ve programmed most recently to see if something is up with the save variables or something 
Since it says "The process cannot access the file because it is being used by another process: “G:\Steam Library\steamapps\common\Stonehearth\saved_games\auto_save\server_state.bin” I’m inclined to think it has to do with some other program interacting with Stonehearth.