Solution : https://service.sap.com/sap/support/notes/350180 (SAP Service marketplace login required)
Key words : 
attached source code corrections, additional key words apo, >>>       release 31imessage class cave   >>>, release 40achoose 'change'message, missing function solution, 006message short text, call program saplcave, apo backorder processing, change sales order, recreate include lcavef04
Related Notes : 
       
| 389313 | Backorder processing: E-message in outbound queue | 
| 375847 | Backorder processing: Control time stamps | 
| 375193 | Backorder processing: Composite SAP note on posting | 
| 321913 | Transfer of the backorder processing results | 
| 321909 | Structure - results of backorder processing |