Daniel Farnier

If only the administration is running HTTPS, featured imaged are not loaded in administration (it is OK for front end).
This code can be used :

function wpdf_ssl_srcset( $wpdf_sources ) {
	if (is_admin()) {
	  foreach ( $wpdf_sources as &$wpdf_source ) {
		$wpdf_source['url'] = set_url_scheme( $wpdf_source['url'], 'httpp' );
	  }	//	fin boucle sur les images
	}	//	fin test si dans l'administration
  return $wpdf_sources;
}
add_filter( 'wp_calculate_image_srcset', 'wpdf_ssl_srcset' );