Object with runtime number '7' not defined

Hi Experts,

I am facing an issue in Production system. In my workflow when an inovice is created based on cost center it will route to the approvers. when an invoice is created with a cost center say 100504 it send to one approver say "A", once he rejects it its sending to initiator for review. if the initiator changed the document with another cost center say 100104 then the workitem is going to another approvers say B. Once the approver B rejects the document the workflow is getting error out. THe error is occuring at send mail step (once it is rejected an external e-mail will be send to initiator).

below is the error description:

EXECUTE_METHOD_OBJECT_SYNC                      Object with runtime number '                  7' is not defined
  EXECUTE_METHOD_BOR                                     Object with runtime number '                  7' is not defined
    EXECUTE_METHOD_BOR                                     Object with runtime number '                  7' is not defined

This is working fine in ERQ, its happening only in PRD. please suggest.

Thanks,

Ranganadh.