Failed to schedule report in Excel format

We have successfully scheduled report in PDF format, however, we go the error message "Error in File D:\Business Objects\BusinessObjects Enterprise 11.5\Data\procSched\eagnmnsx2c5.reportjobserver\~tmp2ee063064fe000e.rpt: The request could not be submitted for background processing" if we select excel as schedule format.

Anyone has any idea why it's happened?

Thank you very much for your help.