On a multi site install, this plugin will fail when saving the openX server location. it redirects to the network homepage on any of the subsites.
that is because on admin.php line 54 the PHP_SELF will never include the path of the site. To fix this, add site_url() . before the $_SERVER['PHP_SELF'] will send the post data of the form to the correct site.
This is what line 54 should look like afterwards.
<FORM name="openxwpwidget_form" method="post" action="<?php echo site_url() . $_SERVER['PHP_SELF']; ?>?page=openx-wp-widget.php&updated=true">