Solution : https://service.sap.com/sap/support/notes/486793 (SAP Service marketplace login required)
Key words : 
/componentdata element/component typeposguid posguid, rule-based availability check, program error solution implement, apo backorder processing, request schedule lines, program correctionscall, sales order, partially delivered, scheduling reason, structure bop_xvbep
Related Notes : 
       
| 487090 | Backorder processing: Duplicate subitems | 
| 375193 | Backorder processing: Composite SAP note on posting |