PowerPivot refresh error on WorkBook using a Data Feed from SharePoint list

Hi,

I'm getting the following error on a workbook's PowerPivot Data Refresh in SharePoint:

Errors in the high-level relational engine. The following exception occurred while the managed IDbCommand interface was being used: The remote server returned an error: (401) Unauthorized.;The logon attempt failed

It is using a Data Feed from a SharePoint List (.atomsvc) I have the Unattended Service Account set to the Farm Account. If I refresh the data from the Excel Client, it works fine.

Any help would be much appreciated.