I'm having the same thing after buying the game [and DLCs] after a Steam sale. The workaround that I found is to start a tutorial game and save it. As odd as it sounds you may NOT be able to load the game you want directly HOWEVER if you load into the tutorial THEN try to load the game you want it may work.
Note: In both of my sandbox playthroughs I've had to use that method. While not great you can often struggle through and finish off a single player game that way.
Edit: Funny things seems to be there are some people getting the same error but a lot of people are pretty stable. Not sure if there's a common thread explaining the problems.