In the SharePoint Server logs do you see the following message?
"MossHost.TryGetWindowsIdentity: Failed to get WindowsIdentity from IClaimsIdentity.
SPSecurityContext.GetWindowsIdentity() threw exception: System.InvalidOperationException: Could not retrieve a valid Windows identity.
---> System.ServiceModel.Security.SecurityAccessDeniedException: Access is denied."
If yes, it may be due to a security issue arising from having your PI Server and SharePoint Server in two different AD domains. The fix is fairly involved and takes a number of changes in network settings and trusts. I would suggest you contact our regular Tech Support and tell them about this issue:
"Sharepoint 2010 Excel Services Accessing PI Data Across Trusted Domains"
They will be able to walk you through the steps required to fix the problem. Hope it helps.
The service "Claims to Windows Token Service" is running as indicated in the document "PI DataLink Server User Guide".
There is a Windows Claims to Windows Token Service and a SharePoint service with basically the same name. They both have to be enabled or you will get this error.
I'm not sure if this is your problem but I had this problem the first time that I tried to set up DataLink server. Either the documentation didn't make it clear that there were 2 separate services to worry about or I just didn't know enough about SharePoint at the time to catch the distinction when I read the documentation.
Hope this helps.