Solution : https://service.sap.com/sap/support/notes/170102 (SAP Service marketplace login required)
Key words : 
choose development class 'scui', -called 'dummy' function modules, sap support portal, syntax check finds, 'not_delete_res' = 'x' parameter, attached source code, function modules'ewo_check_reservation', reservation solution copy, create transaction stfo, call transaction stfo
Related Notes : 
       
| 558430 | ISA/IPC support delay in message processing | 
| 550571 | Internet Sales Support | 
| 508140 | Customer messages - Customer logon data | 
| 31515 | Service connections |