Did you by any chance have a wordpress install in there before you installed C5? Requesting index.html serves a wordpress not found page.

You might have forgotten to remove the .htaccess file that wordpress installs - C5 can have its own version which is totally different, but having the wordpress one will not help.

Remove the .htaccess and rename it (it might not show up with ftp as it's a "hidden" file, in which case just upload an empty one).

Also remove the index.html file if you have one, it may be being picked up instead of index.php, depending on your server configuration