The plugin generates this error on every page when WP_DEBUG is enabled.
Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /var/www/vhosts/redacted/httpdocs/wpms/wp-includes/functions.php on line 3382 Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /var/www/vhosts/redacted/httpdocs/wpms/wp-includes/functions.php on line 3466
You should have WP_DEBUG enabled when developing so that you catch errors like this.
There are also a lot of notices on the Settings page.