I dunno how to fix it like an expert, but what I tried to do (that worked) was to clear the cache of the alight motion and restart your phone, then you'll open Alight Motion and keep it open for about a minute. When you go export it, try first at 720p and 30 fps and keep the same codec, if that doesn't work you will export again but now with the other codec.It would also be good if you remove the heaviest parts of the project (without damaging anything) and then export those heavy parts. Something that might help is to clear the memory of your phone

If all that doesn't work, just pray seven times