Error 500 when consuming NfeDownloadNF

I'm trying to consume the NFE download service on the farm:

link

But I have gotten the following error:

The remote server returned an error: (500) Internal Server Error.

The message I have sent is:

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <soap:Header>
        <nfeCabecMsg xmlns="http://www.portalfiscal.inf.br/nfe/wsdl/nfeDownloadNF">
            <cUF>31</cUF>
            <versaoDados>3.10</versaoDados>
        </nfeCabecMsg>
    </soap:Header>
    <soap:Body>
        <nfeDadosMsg xmlns="http://www.portalfiscal.inf.br/nfe/wsdl/NfeDownloadNF">
            <downloadNFe xmlns="http://www.portalfiscal.inf.br/nfe" versao="3.10">
                <tpAmb>1</tpAmb>
                <xServ>DOWNLOAD NFE</xServ>
                <CNPJ>[NUMERO DO CNPJ]</CNPJ>
                <chNFe>[CHAVE DA NOTA FISCAL]</chNFe>
            </downloadNFe>
        </nfeDadosMsg>
    </soap:Body>
</soap:Envelope>

I've removed all the spaces as requested (I already use some other services successfully)

        request.Method = "POST";
        request.ContentType = "text/xml";

The certificate I'm sure is correct too.

Could someone tell me the error?