The problem is'nt the need for a body tag?
<?xml version="1.0"?>
<Command>
<Security>
<Certificates>
<CA>
<Add><body>-----BEGIN CERTIFICATE-----
MIIF1zCCA4ugAwIBAgIQSMdu9UAsd5VNAivwznsfyzBBBgkqhkiG9w0BAQowNKAP
....
uvDsvjz9WjLU35XmRuPJEV/T90pLR2VCLvwTmAkPHfyWkZzSZaqoNFjsQlWEAUZK
QaAYL89QZy3rsEc=
-----END CERTIFICATE-----</body></Add>
</CA>
</Certificates>
</Security>
</Command>