Dynamics GP - Excel Reports in Sharepoint - The data connection uses Windows Authentication and user credentials could not be delegated. The following connections failed to refresh:

Problem:
You get this error when you try to run a report after you publish an excel report to sharepoint

The data connection uses Windows Authentication and user credentials could not be delegated. The following connections failed to refresh:

Solution:
This solution only works if you have Kerberos enabled.

1. Open SharePoint 2010 Central Administration
2. Navigate to the System Settings section
3. Click Manage services on server
4. Click Start next to Claims to Windows Token Service

Also ensure it is actually running under services.msc

http://www.bidn.com/blogs/BradSchacht/ssis/2406/powerpivot-data-connection-error