Problem using QueryMultipleSchedulesRequest in Javascript

Hello,

I have problem using a soap call in Javascript. What I am trying to do is use the QueryMultipleSchedules message to evaluate the availability of a group of resources (stored in Array). I've fiddled a bit with the format of the fields (especially the dates), but the message now goes trough without errors. Only, the response is always empty. Whatever time block or resource I use, I get an empty xml response where I was expecting to see per sent resource GUID the availability status.

This is the code I use:

//get date parameters
var Start = Xrm.Page.getAttribute("scheduledstart").getValue();
Start = Start.format("yyyy-MM-ddThh:mm:ss.sszzz");
var End = Xrm.Page.getAttribute("scheduledend").getValue();
End = End.format("yyyy-MM-ddThh:mm:ss.sszzz");

var Available = new Array();
Available[0] = "{619FDFC7-2D70-E111-B7D3-1CC1DE086845}";
var TimeCodes = 0;
//assemble xml

/*Generate Soap Body.*/
var soapBody = "<soap:Body>" +"  <Execute xmlns='http://schemas.microsoft.com/crm/2007/WebServices'>" +"    <Request xsi:type=\'QueryMultipleSchedulesRequest\'>" +"       		<ResourceIds>" + Available + "</ResourceIds>" + "       		<Start>" + Start + "</Start>" + "     	<End>" + End + "</End>" + "       		<TimeCodes>" + TimeCodes + "</TimeCodes>" +   "    </Request>" +"  </Execute>" +"</soap:Body>";
alert (soapBody);

/*Wrap the Soap Body in a soap:Envelope.*/
var soapXml = "<soap:Envelope " +"  xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/' " +"  xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' " +"  xmlns:xsd='http://www.w3.org/2001/XMLSchema'>" +
        GenerateAuthenticationHeader() +
        soapBody +"</soap:Envelope>";
/* Create the XMLHTTP object for the execute method.*/
var serverUrl = Xrm.Page.context.getServerUrl() + "/MSCRMservices/2007/crmservice.asmx";
var xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
xmlhttp.open("POST", serverUrl, false);
xmlhttp.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
xmlhttp.setRequestHeader("SOAPAction", "http://schemas.microsoft.com/crm/2007/WebServices/Execute");
/* Send the XMLHTTP object. */
xmlhttp.send(soapXml);
alert (xmlhttp.responseXml.xml);

Which gives me this response:

<?xml version="1.0"?><soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><soap:Body><ExecuteResponse xmlns="http://schemas.microsoft.com/crm/2007/WebServices"><Response xsi:type="QueryMultipleSchedulesResponse"><TimeInfos/></Response></ExecuteResponse></soap:Body></soap:Envelope>

No matter what resources or time values I use, the result always comes up empty, no error, just empty.

Can anyone tell me what I'm doing wrong?

Thanks,
Rimmer