ive had the same error 404 recently, after hours and hours of trying to reinstall scenery, I did notice that when i ran the scenery installer that the previous selected scenery area inwhich i had the error installing did in fact install even tho i did get the error. Try installing scenery a section at a time, thats what i did. I did notice that the error did occur (during the final minute or so of scenery download/install) when I was trying to install the easter europe, russia, asia scenery areas. Hope this helps.