I started up a game and started to do the Spire campaign, but I ran into the issue in the image attached...
Basically, load up the save and select the (unfitted) Spire Generator and you get the error.
I don't know if this is a problem with my install or most installs, though. I'm also not sure how to run an integrity check on the Steam install.
EDIT: Oh, for what it's worth, I don't have Ancient Shadows but I have the previous expansions.