Solution : https://service.sap.com/sap/support/notes/217291 (SAP Service marketplace login required)
Key words : 
additional key words queue error, prerequisites pi99 solution implement, queue status, error message, application error, integration apo->, data transfer, status text, rfc monitor, apo system
Related Notes : 
       
| 321042 | Prod.order: Queue entry 'Connection closed' | 
| 300732 | Display first APO error message in R/3 qRFC monitor |