If the Excel Services global option "Use the EffectiveUserName" is turned on, then all workbooks that use secure store service ID's (SSID) will not refresh data and can not create connections to the SSAS data sources. So effectively the SSID authentication option is not useable if the EffectiveUserName global option is turned on.

ULS entry:

ExternalSource.ValidateConnection: Unable to get a connection: ... Microsoft.Office.Excel.Server.CalculationServer.ExternalSource.TryGetValidatedConnection


The product group is investigating the issue right now, there is an estimated fix date of November 2014.