The construction set started telling me, for no reason I can think of, that it couldn't load the CTL file (whatever that is) and refused to work. A clean re-install fixed the problem, but I still thought I'd ask if anyone knows the cause of this.

Just read http://www.gamesas.com/index.php?/topic/1122534-help-construction-set-will-not-startup-on-my-modded-oblivion/.