PowerPivot - Unable to Refresh

SharePoint 2010 SP1 - SQL 2008 R2 - PowerPivot

I ran across an issue where I was unable to refresh the data in a data slicer. I was receiving the error:

Unable to refresh data for a data connection in the workbook. Try again or contact your system administrator. The following connections failed to refresh: PowerPivot Data 

As it turned out, the account for Claims to Windows Token Service (C2WTS) was set as a domain account.

C2WTS can be set to a domain account, but Kerberos constrained delegation must be configured, as well as the local login policy for that domain account.

The account must be a Local Administrator.  The account must have the following rights in the Local Security Policy User Rights Assignment: Act as part of the operating system; Impersonate a client after authentication; Logon as a service.

Alternatively, the C2WTS account can be set to local system account. - Which is the solution I deployed.

http://technet.microsoft.com/en-us/library/hh231678