what is your exact code?
seems like you are using single quotes;
try to work with double quotes as in the example.
Tried changing the quotes, still doesnt work. Here the code:
$args = array(
'name' => __( 'Sidebar acudame', 'acudame.org' ),
'id' => 'unique-sidebar-id',
'description' => '',
'class' => '',
'before_widget' => '<div class="sidebox">',
'after_widget' => '</div>',
'before_title' => '<h3 class="widget %2$s">',
'after_title' => '</h3>' );
register_sidebar( $args );
the %2$s
only works on the 'before_widget'
parameter.