Getting error 'Redirects to external URLs are not allowed by default'
If you still haven't figured this out, check this comment on d.o: https://www.drupal.org/project/drupal/issues/2612160#comment-11767977

It suggests a workaround by removing the subdir from the script name, adding to settings.php the following:


if (isset($GLOBALS['request']) and
'/subdirectory/index.php' === $GLOBALS['request']-&gt;server-&gt;get('SCRIPT_NAME')) {
$GLOBALS['request']-&gt;server-&gt;set('SCRIPT_NAME', '/index.php');
}