I can reproduce the same exact error by just choosing “Cook” from the “Platforms” menu.
At the moment my best guess is that somehow the builds messed up the assets during an automatic re-import.
I know at one point that I did have Windows turned on as a platform, and an error message that occurred on a previous build led me to a post that said SM5 vs SM6 and Direct X / Windows was an bad combination.
Ugh.
I am thinking I will just have to roll everything back to a known good version in Git and see if I can reconstruct from there.