That path is far too long - $_CONF['path_html'] already contains the /www/... bit.
Change it back to how it was originally:
Text Formatted Code
$_CONF['rdf_file'] = $_CONF['path_html'] . 'backend/geeklog.rdf';
And don't worry about the "geeklog.rdf", Geeklog will use the file name you specify in Content Syndication instead.
bye, Dirk