Make sure the index.html is in the root directory at least. Either the redirector is broke/down or index.html got misfiled.

This citation may suggest some possibilities.

debianhelp - "Enable Custom Error Messages in Apache"

wikipedia says there are issues with using iweb for multiple sites. you could always try using another web page editor with ftp support? my 2 cents there.

may your troubles stay online.

The error you're getting deals with routing. Apache is looking for the default page, be it index.htm(l) or whatever you have the index setup to be. It's also looking for a default 404, which is for page not found.
Routing doesn't necessarily have to do with the file or directory, it could be a host configuration or nameserver issue. hth