I did a complete wipe and reinstall of the game and it fixed the problem. Not sure why!

Thanks to all who commented.

I checked earlier and we're less than 20 turns in and the file size is about 600kb. Didn't seem terrible so I ruled that out.

I've been playing a 5-person game and recently one person surrendered and the AI took over. However, something about the AI corrupts the save file to the point where I can no longer load my turn (it hangs during the loading screen).

I've tried submitting my previous turn and had the other 3 players re-play the latest turn, only to have the same issue reappear. Is there anything I can do or will I have to start a new game?