The error is the email templates after of translate, edit and cause the cart show that message in order confirmation (Cart cannot be loaded or an order has already been placed using this cart). Also the system es very slow to place an order and for any related action to send a email.
When you changed a status order or register a new user, you can try disable the email funtion in the prestashop preferences for the fast solution or edit the file public_html/classesPaymentModule.php and delete the function Mail:: and this disable email order confirmation.
After this modification your prestashop order process very fast.
I am updating more info...
Sorry for my english
DELETE THIS FUCTION
Mail::Send(
(int)$order->id_lang,
'voucher',
sprintf(Mail::l('New voucher for your order %s', (int)$order->id_lang), $order->reference),
$params,
$this->context->customer->email,
$this->context->customer->firstname.' '.$this->context->customer->lastname,
null, null, null, null, _PS_MAIL_DIR_, false, (int)$order->id_shop