Solution : https://service.sap.com/sap/support/notes/484343 (SAP Service marketplace login required)
Key words : 
program error solution correct, rule-based availability check, material availability date, apo backorder processing, request schedule lines, rule evaluation, sales order, symptom occurs, main items, prerequisites
Related Notes : 
       
| 487090 | Backorder processing: Duplicate subitems | 
| 375193 | Backorder processing: Composite SAP note on posting |