Solution : https://service.sap.com/sap/support/notes/435343 (SAP Service marketplace login required)
Key words : 
additional key words transaction, rules-based atp check, interactive backorder processing, error message 'item, /sapapo/bopibackorder processing, source code correction, entries hang, problem occurs, solution implement, prerequisites
Related Notes : 
       
| 451511 | BOP: Postprocessing is not posted completely | 
| 439745 | BOPI: New delivery schedules / scheduling | 
| 435292 | BOPI: Changes to subitems are not transferred | 
| 431807 | BOPI: Incorrect data transfer, COM error | 
| 375193 | Backorder processing: Composite SAP note on posting |