EWS Schema information not found

I am trying to connect an Xamarin.Android C# program to an Exchange 2013 server via Exchange WebService (EWS). I send the following XML file (dynamically created using XmlTextWriter):

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://ift.tt/ra1lAU" xmlns:soap="http://ift.tt/sVJIaE">
 <soap:Header>
  <t:RequestServerVersion Version="Exchange2007_SP1" xmlns:t="http://ift.tt/rNGrBu" />
 </soap:Header>
 <soap:Body>
  <m:GetRoomLists xmlns:m="http://ift.tt/rPEB4w" />
 </soap:Body>
</soap:Envelope>

and receive this answer (minus formatting):

<?xml version="1.0" encoding="utf-8"?>
<s:Envelope xmlns:s="http://ift.tt/sVJIaE">
 <s:Body>
  <s:Fault>
   <faultcode xmlns:a="http://ift.tt/rNGrBu">a:ErrorSchemaValidation</faultcode>
   <faultstring xml:lang="de-DE">Fehler bei der Schemaüberprüfung der Anforderung: Die Schemainformationen für das Element 'http://ift.tt/1zn0hMz' konnten nicht gefunden werden.</faultstring>
   <detail>
    <e:ResponseCode xmlns:e="http://ift.tt/KCNDGg">ErrorSchemaValidation</e:ResponseCode>
    <e:Message xmlns:e="http://ift.tt/KCNDGg">Fehler bei der Schemaüberprüfung der Anforderung.</e:Message>
    <t:MessageXml xmlns:t="http://ift.tt/rNGrBu">
     <t:LineNumber>7</t:LineNumber>
     <t:LinePosition>6</t:LinePosition>
     <t:Violation>Die Schemainformationen für das Element 'http://ift.tt/1zn0hMz' konnten nicht gefunden werden.</t:Violation>
    </t:MessageXml>
   </detail>
  </s:Fault>
 </s:Body>
</s:Envelope>

The error texts translate roughly to "Error while checking request scheme" (<e:Message>) and "Scheme information for element '<URL>:GetRoomLists' could not be found" (<t:Violation>); <faultstring> is the combination of both.

I don't understand where the error lies. Is my request XML bad, is the Exchange Server misconfigured or is a module missing (if so, which one?), is there some other error?

I've tried several other things in the SOAP body, e.g. <GetRooms><RoomList><EmailAddress>room@server</EmailAddress></RoomList></GetRooms>, with the same error message (complaining about GetRooms).