How to Delete a Specific Invitee on an Event

Hi,

Can someone please help me resolve the error I encountered on deleting a specific contact on an Event. I used "ContactId" as the identifier of the contact record to be deleted on the list of invitees on an event. Yet I encountered the error  below. Isn't that ContactId is the same as the Row Id on Contact records?

Result Set:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
   <SOAP-ENV:Body>
      <SOAP-ENV:Fault>
         <faultcode>SOAP-ENV:Server</faultcode>
         <faultstring>No user key can be used for the Integration Component instance 'MedEd'.(SBL-EAI-04397)</faultstring>
         <detail>
            <siebelf:siebdetail xmlns:siebelf="http://www.siebel.com/ws/fault