You'll have to remplace %%excerpt%% in /inc/wpseo-functions.php with
'%%excerpt%%' => ( !empty( $r->post_excerpt ) ) ? strip_tags( $r->post_excerpt ) : substr( strip_shortcodes( strip_tags( $r->post_content ) ), 0, 155 ),
Yoast this problem exist since 3 month and must be fixed at each update. When this problem will be fixed ?
