I have the same problem, my iOS build cannot load any scene or script when testing in a physical iPhone. (the export worked before Godot 3.5.1)
In my case, it was because a material was being imported Uppercase, but the file itself was lowercase. It looks like MacOS is not case sensitive, but iOS is.
After manually renaming the material (outside of Godot), everything worked properly.
PD: What happens with Godot error output is that, if something fails to import, it starts a chain of import errors. Check the very first import error you are having, it is probably the real source of the problem.