I have powerpivot files in SP2013 that produce "An error occured while working on the data model in the workbook" when a sss id or none is specified for the authentication in excel connection properties. The powerpivot works fine if I use windows authentication. This is the ULS log:
DataModelManager.GetModelAndMarkUsed: failed to create connection, ex: Microsoft.Office.Excel.Server.CalculationServer.Interop.ConnectionException: Exception of type 'Microsoft.Office.Excel.Server.CalculationServer.Interop.ConnectionException' was thrown.
ConnectionManager.GetModelConnection: Invalid connection info: [Oledb, Stored, Excel, False, , ShouldUseEffectiveUsername: False]
WebServiceHelperMethods.ExecuteWebServiceRequestHandler: completed processing 'ApplySlicerSelection' web method call with errors. [MessageId: GenericModelFailure, MessageIdHash: 1642688310]
Scheduled refreshes on the powerpivot w win auth works fine. Scheduled refresh on the sss id/none show "A schedule cannot be enabled for a workbook with no external data sources".