Solution : https://service.sap.com/sap/support/notes/632624 (SAP Service marketplace login required)
Key words : 
apo backorder processing, local atp check, 'type' = 'sap_apo' entry, material plant combination, transferred correctly, terms include, ndisystemsfunction module, cave_sl_doc_inbound_bop reason, program error, situations arises
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting |