Hey all,
I've been seeing some issues related to save files over the last few months, and they seem to be related to Steam's Cloud Save feature. It's only affecting a few users, but when it happens, it breaks the game until you delete your cloud save file and/or your game's save data.
The problem's more likely to be in my code than in Steam's, but I've disabled Steam Cloud Save while I investigate. This shouldn't affect your gameplay at all: the game will still save and load your progress. It just means that if you switch to a different computer, it won't carry your progress across to it.
I'll keep you updated - thanks for your patience. :)