Hello together,

I have a tricky issue and hope that you can help me:

Problem: I want to build a Reporting Services Report with a PowerPivot datasource with Report Builder 3.0. When I try to access the datasource via Report Builder I get the error: The remote server returned an error: 401 - Unauthorized.
The PowerPivot file lies on a Sharepoint 2013 with Kerberos authentification.

Logs: Here are some sample log entries after getting the error:
Token Cache: Could not find SPSite for 'sharepoint.com/_vti_bin/PowerPivot/secureredirector.svc/?DataSource=sample.xlsx'.

Exception trying get context compatibility level: System.IO.FileNotFoundException: The site sharepoint.com/_vti_bin/PowerPivot/secureredirector.svc/?DataSource=sample.xlsx could not be found in the Web application SPWebApplication Name=sharepoint.com. 

Non-OAuth request. IsAuthenticated=True, UserIdentityName=0#.w|user, ClaimsCount=118

Notes: I tried some solutions from the internet, where the web.config from PowerPivot has to be changed. Also I saw that it could be Kerberos related, but no detailed information where to start as the other features are working fine. SPNs and Delegations are configured. And changing to NTLM is not an option as this is a multi-server platform. 

I hope that you have another idea, where the problem could be.

Kind regards, 
Robin