Somehow the production and training times are getting screwed up, resulting in negative numbers. The save starts with a barracks building and a food shortage. If you build an Apiary on the Twilight Bees just to the left of the city, within 3 turns the Scouts will build after reaching a negative build time. If you stay in food shortage they will build normally.

scouts_pre_negative.EleSav

I also had it happen earlier in the game with building a Holsten's Library. Although I do not have the save leading up to the glitch, I do have a save that shows it in action. (Interestingly if you cancel the build while it is in the negative it will not let you rebuild it.)

holsten_negative.EleSav