IDoc updating PO (STO) item with blank delivery date

Hi,

We have PORDCH and SHP_OBDLV_CREATE_STO IDocs that update STO (PO) items (IS Retail)

However randomly it seems to blank out the existing delivery date in the PO line item. It also does this to the schedule line items, again randomly. PORDCH IDoc does not even have a delivery date in it, it just wants to flag the STO as delivery complete. About one third IDocs daily suffer this error.

I think BAPI BAPI_PO_CHANGE gets invoked somewhere but there are no enhancements/mods so it is standard application . A majority of theIDocs process correctly but one third don't because of the blanked out delivery date. There is no fixed pattern. PO gets committed. However since the delivery date is blank, it has to be manually corrected. The status record in the IDoc says 'Please enter a delivery date' or 'Purchase order item NNNNN still contains faulty schedule lines'.

Please help!

Thanks and regards,

Mario