Solution : https://service.sap.com/sap/support/notes/503793 (SAP Service marketplace login required)
Key words : 
solution implement source code corrections, rule-based atp subitems occurred, manual atp check, remaining main item, 'parameters check' tab, backorder processing result, /sapapo/bop_resultbackorder processing, rule evaluation, backorder processing, /sapapo/bop
Related Notes : 
       
| 505811 | |
| 375193 | Backorder processing: Composite SAP note on posting |