If the registration API of the wordpress plugin WPML / OTGS returns an error 403 or "invalid response" try to add an user-agent header to the api call.
wp_remote_post()
seems to send w/o user-agent - perhaps depending on your installation/server-config.
Add this php snippet to your functions.php or some custom plugin:
function my_http_request_args($args) {
$args['user-agent'] = 'WordPress';
return $args;