Same thing is happening to me, every single fix suggested on here does not work. I have triggered the same thing to occur in a completely different way by repeating Rank 6 mission after dieing during completeing it only gave half credit. It allowed me to progress onto ZM and finish getting ZM15. Years later I tried to get Rank 7, would not let me get it. Repeating rank 6 has erased my ZM progress, but I still retain completed list of missions and all KI's from missions. After completing ZM all the way through a second time exactly to the point where you enter sky....... it stillllllllll wont let me into the sky door. I have never changed nations.