If you downloaded last night's 3.1.0.0, please restart Steam to download 3.1.0.0.1 which replaces the game exe (and fixes the crash introduced in 3.1.0.0).
Apologies for that! Short version - I tried to replace the exe that was used for Steam Achievements and was alerted just a few moments ago that was a mistake. That change has been reverted, so we'll still see the extra pop-up text box from time to time, but at least the game will launch. I'll look into a more permanent solution for that pop-up box in the future.