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' );