Phil

Made it work:

$posts = get_posts( array(
        'post_type'       => array( 'event', 'post'),
        'posts_per_page'  => 9,
	'orderby'          => 'date',
	'order'            => 'DESC',
	'tax_query'      => array(
	    'relation'   => 'OR',
            array(
                'taxonomy'  => 'post_tag',
                'field'     => 'slug',
                'terms'     => array('featured', 'featured-de'),
            ),
            array(
                'taxonomy'  => 'event-tags',
                'field'     => 'slug',
                'terms'     => array('featured'),
            ),
        )
) );

Thank you for looking into this!

Unfortunately I still couldn’t make it work.
I will follow your suggestion and move the topic to the plugin forum…