Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
The error 404 (file not found) informs the user that the page specified by the URL does not exist on the web server (at least not in the location specified).
Make sure that you have uploaded your files to the correct folder on your web server.
In order for files to be publicly accessible on the internet, they should be uploaded to a folder which may be named 'public_html' or 'htdocs' (though your server may be different). Please check with your web host if unsure.
I uploaded my test folder to the server, when I click the menu to one gallery, it shows the following.
It may just be that the link to your gallery's HTML index page is incorrect. Check the link with regard to the path used and the actual HTML index page name.
I do not know the directory structure of your web server and I do not have the URL to your web page but it looks like your link might need to be changed from:
SimpleViewer Support Team.