I suspected it: Fusion is not like it used to be, here’s an extensive, still euphoric but critical review by Roberdo.
Fatal error: Uncaught Error: Call to undefined function dsq_is_installed() in /var/www/ud06_61/html/wp/wordpress/wp-content/plugins/force-ping-display-with-disqus/forceping.php:15 Stack trace: #0 /var/www/ud06_61/html/wp/wordpress/wp-includes/class-wp-hook.php(286): ts_show_pings('/var/www/ud06_6...') #1 /var/www/ud06_61/html/wp/wordpress/wp-includes/plugin.php(203): WP_Hook->apply_filters('/var/www/ud06_6...', Array) #2 /var/www/ud06_61/html/wp/wordpress/wp-includes/comment-template.php(1470): apply_filters('comments_templa...', '/var/www/ud06_6...') #3 /var/www/ud06_61/html/wp/wordpress/wp-content/themes/waipoua/single.php(18): comments_template('/comments.php', true) #4 /var/www/ud06_61/html/wp/wordpress/wp-includes/template-loader.php(74): include('/var/www/ud06_6...') #5 /var/www/ud06_61/html/wp/wordpress/wp-blog-header.php(19): require_once('/var/www/ud06_6...') #6 /var/www/ud06_61/html/wp/wordpress/index.php(17): require('/var/www/ud06_6...') #7 {main} thrown in /var/www/ud06_61/html/wp/wordpress/wp-content/plugins/force-ping-display-with-disqus/forceping.php on line 15