What does Unknown logical object 'Sun' might prevent other objects from loading. Rename it to '__Sun' if you do not want to export it. mean?

The engine doesn't seem to like objects without special meanings. As soon as it encounters such a object it just stops loading at that point even if there is still something to load.
If you want to create a helper object that shouldn't be exported just name it with two underscores at the beginning, like "__Sun".  The object itself and all it's children will be ignored.