Comment 9 for bug 823783

I'm having the same problem. The debug logs are indicating that ReferenceItemID is set to null...

It's sending:
    <m:CreateItem SendMeetingInvitations="SendToNone" MessageDisposition="SendAndSaveCopy">
      <m:Items>
        <t:AcceptItem>
          <t:ReferenceItemId Id="null" ChangeKey="2"/> ...

And getting the following error back:
<m:CreateItemResponseMessage ResponseClass="Error">
          <m:MessageText>Id is malformed.</m:MessageText>
          <m:ResponseCode>ErrorInvalidIdMalformed</m:ResponseCode>
          <m:DescriptiveLinkKey>0</m:DescriptiveLinkKey>
          <m:Items/>

My emails are being read from imap via a proxy - not sure if that could be a problem?