Solution : https://service.sap.com/sap/support/notes/899982 (SAP Service marketplace login required)
Key words : 
alm bapi downgrade kit, pm/cs order, relevant correction instructions, bapi_alm_order_maintain bapi, program correction, incorrectly receive, error message, terms saplnwbt, afabd-aplzl_nch, afabd-aplzl_vor
Related Notes : 
       
| 1007368 | Changing relationship not possible using BAPI II | 
| 708508 | Downgrade BAPI_ALM_ORDER_MAINTAIN -> errors | 
| 633987 | Relationships maintenance prevented w/ incorrect msg. CN814 | 
| 539597 | ALM order BAPIs: Error when maintaining relationship | 
| 537755 | BAPI_ALM_ORDER_MAINTAIN: Relationship changes not saved | 
| 525056 | Not possible to edit relationships |