Output assigned custom taxonomy to custom post type

Here’s the line that causes your problem:

$terms = get_terms( 'serie' );

You use get_terms function which retrieves the terms in a given taxonomy.

And you want to get terms assigned to given post, so you should use get_the_terms which retrieves the terms of the taxonomy that are attached to the post.

So change the line above to:

$terms = get_the_terms( get_the_ID(), 'serie' );