"Id is Malformed" error when using Buttons in message view

Accepting a Meeting via buttons in message view leads to "Id is Malformed" error -
see also info in last lines of bug 601747.

The following is the request created by pressing "accept button" and the answer from EWS...

41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: PS: ACCEPTED
41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: doRequest
41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: Kick: processing requests: false queue: 1
41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: Sending: <?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages">
  <soap:Header>
    <t:RequestServerVersion Version="Exchange2007_SP1"/>
  </soap:Header>
  <soap:Body>
    <m:CreateItem SendMeetingInvitations="SendToNone" MessageDisposition="SendAndSaveCopy">
      <m:Items>
        <t:AcceptItem>
          <t:ReferenceItemId Id="null" ChangeKey="1"/>
        </t:AcceptItem>
      </m:Items>
    </m:CreateItem>
  </soap:Body>
</soap:Envelope>
41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: getInterface
41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: IID: {d974c99e-4148-4df9-8d98-de834a2f6462}
41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: reply

41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: <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:Header>
    <t:ServerVersionInfo MajorVersion="8" MinorVersion="2" MajorBuildNumber="176" MinorBuildNumber="2" Version="Exchange2007_SP1" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types"/>
  </soap:Header>
  <soap:Body>
    <m:CreateItemResponse xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages">
      <m:ResponseMessages>
        <m:CreateItemResponseMessage ResponseClass="Error">
          <m:MessageText>Id is malformed.</m:MessageText>
          <m:ResponseCode>ErrorInvalidIdMalformed</m:ResponseCode>
          <m:DescriptiveLinkKey>0</m:DescriptiveLinkKey>
          <m:Items/>
        </m:CreateItemResponseMessage>
      </m:ResponseMessages>
    </m:CreateItemResponse>
  </soap:Body>
</soap:Envelope>

41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: Error: ErrorInvalidIdMalformed msg: Id is malformed.
41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: fail status: 200 Id is malformed.