Glad You found the problem! (and sorry if We led you on a wild goose chase, guess You should have just went with Your hunch and checked that first, but you always want to check the basics first right).
Thanks for the pic too, those later model sub-frames look less swiss-cheezed. I have a later earlier-model one also that does not have the top-most oval hole my old one broke around, it would be nice to have the pedals a bit higher too though, oh, and a pedal-crank shaft diameter that accepts standard arms...
(Scotts late-model subframe linked from above)