<order_id>, <key> is known already.

<?xml version="1.0" encoding="UTF-8"?>
<prestashop xmlns:xlink="http://www.w3.org/1999/xlink">
  <orders>
     <order>
       <current_state xlink:href="https://myshop.com/api/order_states/5">
           <![CDATA[5]]>
       </current_state>
     </order>
   </orders>
</prestashop>

<?xml version="1.0" encoding="UTF-8"?>
<prestashop xmlns:xlink="http://www.w3.org/1999/xlink">
    <errors>
        <error>
            <message>
                <![CDATA[Internal error. To see this error please display the PHP errors.]]>
            </message>
        </error>
    </errors>
</prestashop>