The data connection uses Windows Authentication and user credentials could not be delegated
This is one of the two main errors that users could see from Excel Services when using PowerPivot. This is encountered when refreshing PowerPivot data connections or performing an action which requires re-querying the PowerPivot database, such as clicking on a slicer or expanding a node in a pivot table. To debug, some level of understanding of what Excel Services is doing is required. For this, I recommend reading an earlier post on this blog by Dave …http://powerpivotgeek.com/2009/12/11/excel-services-delegation/ . In general, this is an add-on to Dave’s post which is a quick summary of how to debug this error.