I have added some fields on the service activity form and when i now when i add a service activity (though the contact) and save i get the error messge -General Failure in Scheduling Engine

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: activity.ServiceIdDetail:
<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">
  <ErrorCode>-2147220989</ErrorCode>
  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
  <Message>activity.ServiceId</Message>
  <Timestamp>2012-05-22T11:22:53.9421758Z</Timestamp>
  <InnerFault i:nil="true" />
  <TraceText i:nil="true" />
</OrganizationSviceFault>

Can anyone tell me what this is and how i fix it?

Thanks

Vikki