Meta Box

Yes, im using rwmb_meta().

Creating the field:

function kie_jobs_aufgaben_metabox( $meta_boxes ) {

    $prefix = 'kie_';

    $meta_boxes[] = array(
        'id' => 'aufgaben',
        'title' => esc_html__( 'Aufgaben', 'kie-stellenboerse' ),
      'post_types' => array( 'jobs' ),
        'context' => 'after_title',
      'priority' => 'high',
        'autosave' => true,
      'fields' => array(
        array(
            'name'    => 'Aufgaben',
            'id'      => $prefix . 'aufgaben',
            'type'    => 'wysiwyg',
            'add_to_wpseo_analysis' => true,
            'class' => 'kie_metabox_textarea',
            'raw'     => false,
            'options' => array(
                'textarea_rows' => 4,
                'teeny'         => true,
            ),
        ),
      ),
    );

    return $meta_boxes;
}

Updating Post Meta:
update_post_meta($post_id, 'kie_aufgaben', wp_kses_post($aufgaben), false);

Thanks for the fast reply. You can close this topic. I found what was causing the error - it was not meta box!

Thank you!