As I said in my update, it was a savegame error.
Additional Note: The FNV DLC quests except the one for Lonesome Road will not start when I begin with FO3. Is that normal? I'm guessing that they don't appear until I travel to the Mojave? Well, if they don't start after traveling miles away from DC, then maybe console commands would do the trick. The console commands may not work for the FO3 assets, but they do work for the ones from FNV. I'm not sure if this is related to the load order.