Delete Backend Reservation not possible

Hello gurus,

my customer will delete a shopping cart position with a reservation in backend. So it works fine and die sc gets the status 'follow-on document deleted'. But the deletion-flag in the backend reservation is not set! I've debugged the function calls to ERP fm BBP_RESERVATION_DELETE.

FUNCTION BBP_RESERVATION_DELETE.
*"----------------------------------------------------------------------
*"*"Lokale Schnittstelle:
*"       IMPORTING
*"             VALUE(RESERVATION) LIKE  BBPRKPF-RES_NO
*"       TABLES
*"              RESERVATION_ITEMS_FOR_DELETE STRUCTURE  BBPRESB
*"              RETURN STRUCTURE  BAPIRETURN OPTIONAL
*"----------------------------------------------------------------------
   DATA: LT_CURRENT_RESB LIKE RESB  OCCURS 0,
         LT_RESB         LIKE RESB  OCCURS 0,
         LT_ZRESB        LIKE RESB  OCCURS 0,
         LT_DIS          LIKE MDISP OCCURS 0,
         LV_SUCCESS      TYPE SYSUBRC.
   DATA: lv_external_call TYPE sap_bool.

   CALL METHOD cl_rfc=>check_rfc_external
     RECEIVING
       external_call    = lv_external_call
     EXCEPTIONS
       kernel_too_old   = 1
       unexpected_error = 2
       OTHERS           = 3.

   IF lv_external_call = abap_true.
     EXIT.
   ENDIF.

   CLEAR: RETURN, RETURN[].

Can someone explain why the fm denies external calls while it is declared as RFC fm?

BR

Danilo