Solution : https://service.sap.com/sap/support/notes/537755 (SAP Service marketplace login required)
Key words :
function module ibapi_alm_order_post, table parameter it_orders, 'optional' = 'x' flag, save relationships, service orders, maintenance orders, bapi bapi_alm_order_maintain, terms bapi, ordermaintain reason, program error
Related Notes :
| 899982 | You cannot change relationship using BAPI |
| 736285 | Notes for downgrade kit ALMBAPI II |
| 708508 | Downgrade BAPI_ALM_ORDER_MAINTAIN -> errors |