custom post template file not shown, instead all the time 404.php

Question

i'm completely confused, want to use my own template file for a custom post type and of course i read all about this here or in the Codex (http://codex.wordpress.org/Post_Types#Custom_Post_Type_Templates )

Also i found users with the same problem ( https://stackoverflow.com/questions/14461285/what-would-make-a-custom-post-type-template-not-show-up ), and of course i already have updated the permalink page (change the setting > save > change setting to original back > save !). Nothing has worked out so far :( .

What did i done ?

First i register my custom post type as usual with that code:

register_post_type( 'series', array(
      'supports'                => array('title', 'editor'),
      'menu_position'           => 20,
      'show_ui'                 => true,
      'show_in_nav_menus'       => false,
      'show_in_menu'            => true,
      'public'                  => true,
      'register_meta_box_cb'    => array($this, 'addMetaBoxes'),
      'public_queryable'        => true,
      'has_archive'             => true
    )
);

After the registration i expected to have access to my custom template file, if i create the single-series.php template file inside my theme root directory.

But ... boohhh.

All the time my 404.php is called instead of my desired custom template file... what (the hell) goes wrong ??? Do i forget something important ? ...argghh ! Please help.

Thanks in advance!