Since I work with Navtweak, I also encountered numerous crashes when saving the navboot file. I think this is a mood of Navtweak, e.g. it crashes, afterwards I try to do the same modifications again and it saves the file without an error. I also noticed that you should keep the swapped files as small as possible or at least not much bigger than the original ones. The biggest size difference is probably found at the splashscreen, so try to reedit it that it gets smaller (in filesize). I found a nice looking splashscreen on the internet which was about 60kB in size, after editing it's about 7kB and looks exactly the same, but now Navtweak crashes less. Also, I'm now saving navboot after every single modification (splashscreen, text strings, ...) as a new file so that I don't have to do all the mods again when Navtweak is not in the mood to cooperate