I think it's already declared as a default index in this line in .htaccess:
<b>DirectoryIndex index.php</b> It's a stock-standard .htaccess so it shouldn't be a problem... The directory browsing (listing) is disabled - I edited /usr/local/apache/conf/access.conf and removed the word "Indexes". After editing it looked like this:
<Directory />
Options FollowSymLinks ExecCGI Includes
AllowOverride All
order allow,deny
allow from all </Directory>
This is not really EZ-related at all, but first off, whenever your webserver throws error codes at you check the server logs for more details! Make sure your Apache-configuration allows the use of .htaccess files, check file/directory permissions and you may also want to consult the Apache documentation at http://httpd.apache.org/docs-project/ .