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

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!