Solution : https://service.sap.com/sap/support/notes/644015 (SAP Service marketplace login required)
Summary : 
      This SAP Note addresses the error RSAR 051 during an RFC call to an external server, enabling exception reporting in the monitoring system. The error stems from a programming issue. For correction, depending on your BW system release, you can import specific Support Packages as described: For BW 3.0B, import Support Package 15 (SAPKW30B15) following details in note 571698. For BW 3.1 Content, import Support Package 9 (SAPKW31009), as per details in note 571720. Advance information may be available before the Support Package release, marked as a "preliminary version".
Key words : 
note corrects error message rsar 051, 1 content            import support package 9, 0b            import support package 15, 1 content support package 09, terms rsar051 reason, attached correction instructions, 0b support package 15, external server program, bw support packages, program error
Related Notes : 
       
| 571720 | SAPBWNews BW 3.1 Content Support Package 09 | 
| 571698 | SAPBWNews BW 3.0B Support Package 15 |