you usually get this error if the information is not in the shape as expected by SAP.
An IDoc does not have a user interface, it is a technical medium, hence conversion exits that are in place for manual entries are not run through if you post IDocs.
IF the field for the delivery note number is 10 digit then and your number only 5 then you have to have leading zeros in the IDoc and not just the 5 digits a users sees in the screen.
