I have partly figured it out! Either FT3 continues to write to the file even though the dialogue says it's done, or it keeps a lock on it, or something. But after letting the file "cook" for a few minutes (even though the file size was not increasing) I could open it in Wilbur. It also seems to be extremely touchy about the exact view in FT3 - just by moving the globe one direction or the other, I can create an mdr what won't open in Wilbur, even though the previous one does.