your code is causing validation errors: http://validator.w3.org/check?uri=http%3A%2F%2Fwww.opensolution.gr%2F&charset=%28detect+automatically%29&doctype=Inline&group=0
http://codex.wordpress.org/Validating_a_Website
move the new code to a location after the <body> tag, possibly to just after the outermost div .main-wrapper.
your new 'sidebar' outputs li tags without surrounding ul tags;
therefore either change the register code, for instance to:
register_sidebar( array(
'name' => __( 'header-widget'),
'id' => 'mycustomwidgetarea',
'description' => __( 'An optional widget area for your site footer', 'twentyeleven' ),
'before_widget' => '<div id="%1$s" class="widget-container %2$s">',
'after_widget' => '</div>',
'before_title' => '<h3 class="widget-title">',
'after_title' => '</h3>',
) );or change the 'sidebar' code, for instance to:
<?php if( is_active_sidebar('mycustomwidgetarea') : ?>
<ul><?php
// Custom widget Area Start
dynamic_sidebar('mycustomwidgetarea');
// Custom widget Area End
?>
</ul>
<?php endif; ?>might need adjustments of the formatting.