If you're running OpenSEF for Joomla! and are getting errors when you enable OpenSEF in the configuration panel that reference "URL file-access is disabled in the server configuration," be sure to edit your .htaccess file properly. This really doesn't seem to be mentioned in any of the Joomla! or OpenSEF documentation that exists, but there are two different SEF sections in .htaccess. One is for Joomla! Core SEF only, the other is for 3rd party SEF. You need to comment out the "Joomla! core SEF Section" and uncomment the "3rd Party SEF Section" to get OpenSEF to work properly.