I just installed this plugin 3 days ago and wanted to create the sitemap every day at 6:00 am. However when I tried to reach the URL given in the setup part I got fopen errors on wp-config.php.
When I looked into the cron.php file I found the following:
require_once($_SERVER["DOCUMENT_ROOT"] . "/wp-config.php");
The $_SERVER["DOCUMENT_ROOT"] does not give the entire path to the WordPress installation directory. In my case the installation of WordPress is done in a subfolder domain.com/wordpress/. I just changed this line to:
Since WordPress knows the following structure; wordpressroot/wp-content/plugins/strictly-google-sitemap/
I suppose more people will get onto this problem.