Solution : https://service.sap.com/sap/support/notes/730796 (SAP Service marketplace login required)
Key words : 
temporary quantity assignments, incorrect items remains, apo backorder processing, backorder processing remains, backorder processing, errors occur, sales document, error message, symptoms occur, program error
Related Notes : 
       
| 1161134 | |
| 609435 | Composite SAP note: Performance in backorder processing | 
| 375193 | Backorder processing: Composite SAP note on posting |