Answer
Check the BA_EXECUTION_SERVER_AVAILABLE System Parameter on the BA System Parameters screen — it has three distinct settings that directly control this behavior. 'NO': both On-Demand and Scheduled reports need the BA Excel Plugin installed locally, with On-Demand opening directly in Excel. 'FOR_ALL_INFO_SERVICES_REPORTS': neither On-Demand nor Scheduled reports need the plugin at all, since both end up in the report archive instead of opening directly in Excel. 'ONLY_FOR_SCHEDULED_REPORTS': On-Demand reports still need the plugin locally installed and open directly in Excel, but Scheduled reports don't need it, ending up in the archive instead. The assumption that the BA Execution Server alone eliminates the need for the local Excel Add-in is only actually true for On-Demand reports specifically under the 'FOR_ALL_INFO_SERVICES_REPORTS' setting — any other setting still requires the add-in for at least some report types, which is exactly what produces this blank-Excel-output symptom for end users lacking it.