Well, I finished my game with Norway last night. Then I went so far as to completely delete the civ and reinstall it from the OP, but I'm still have the same problem with the Birkebeiner. It still doesn't upgrade to anything and becomes available at Metal Casting. The only other MODS I had loaded were InfoAddict and Comunitas Map (which I wasn't using). I can't see how either of those would affect this situation, but I'm not a modder, so who knows.