Solution : https://service.sap.com/sap/support/notes/394539 (SAP Service marketplace login required)
Key words : 
additional key words transaction, tab 'check parameter', program /sapapo/lbop_bufferfb0, /sapapo/bop_updatebackorder processing, /sapapo/bop, backorder processing, symptom due, checked explicitly, problem occurs, rule evaluation
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting |