IDOC Status 51 for HTTP to SAP ECC

Hello Experts,

I have a HTTP to IDOC Scenario , its running fine for messages. Some messages are failing in ECC with the IDOC status as 51 ,  with the error that it was using the User as different one for the failed ones. But for successful one it was using the correct one which we have maintained in RFC destination . Can some one suggest me what needs to be done.

Thanks,
Vijay Kumar