Probably i’ve found the problem, the style to override is
#wp-admin-bar-wp-logo > .ab-item .ab-icon
No more the
#header-logo
In the meantime that the plugin is fixed i’ve used this code into functions.php:
// ADMIN CUSTOM LOGO
function custom_logo() {
echo '<style type="text/css">
#wp-admin-bar-wp-logo > .ab-item .ab-icon { background-image: url('.get_bloginfo('template_directory').'/images/icon-header.png) !important; background-position: 0 0 !important;}
#wpadminbar.nojs #wp-admin-bar-wp-logo:hover > .ab.item -ab-icon, #wpadminbar #wp-admin-bar-wp-logo.hover > .ab-item .ab-icon{background-position: 0 -16px !important;}
</style>';
}
add_action('admin_head', 'custom_logo');