When you get a message like this it is normally two fold. The first part is probably occurring as a result of some security that you have in place on the site, like htaccess or something like that.
The second part of the error talking about the 404 message means that the custom error page that it normally uses was not found.
You should check the permissions you have on the directory you are uploading to and also check to ensure that there is no directory or other security applied to that folder or that part of the site.