Problems with the WP_Query query for a Custom post type

Tax_Query should be an array of arrays. Right now you have it is a single-dimension array. Can you try:

$the_query = new WP_Query( array(
    'post_type'         =>  'circulares',
    'posts_per_page'    =>  2,
    'tax_query'         =>  array(
                              array(
                                'taxonomy'      =>  'curso',
                                'field'         =>  'slug',
                                'terms'         =>  'tercero',
                             ))
));

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