WPMU DEV Plugin

Recently I purchased, installed and activated WPMU DEV and the WPMU DEV Dashboard on my WordPress theme. I only need basic content protection so this plugin was great for just that. Initially everything was working just fine but after a few days it stopped working. After taking a look at my plugin page on my WordPress Dashboard, I found this error message:

Warning: Missing argument 2 for wpdb::prepare(), called in /home/dhenry1/public_html/wp-content/plugins/membership/membershipincludes/classes/class.adminbar.php on line 77 and defined in /home/dhenry1/public_html/wp-includes/wp-db.php on line 990

Now, I really don’t know much about WordPress and I certainly don’t know what this means. Can someone please explain what this is and how to possibly fix it? Any help would be greatly appreciated. Thank you.