Ilaria,
I've run into this before. usually, to fix it, i've had to include:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:glob="http://sap.com/xi/SAPGlobal20/Global">
<soapenv:Header/>
<soapenv:Body>
before the XML you have currently, and
</soapenv:Body>
</soapenv:Envelope>
after.
