warning about missing argument

several days ago strange things started to happen: I see warning on frontend (!!!) that says: “warning! missing argument 2 on line 290 in class-visual-subtitle.php” and this warning shows full path to that file location on my server even if I am not logged in my wordpress site! Panic! Help!

http://wordpress.org/extend/plugins/visual-subtitle/