We recently created a new web application on our SharePoint farm, we chose claims-based authentication as authentication option. All seemed ok until i wanted to create a report on top of the PowerPivot application in SSRS. I got a error “‘Too many automatic redirections were attempted” in the report builder 3. Searching on the Internet i can across a blog of Dave Wickert that explains that we need to run a PowerPivot web application in classic mode: Why PowerPivot requires ‘classic-mode’ web applications.
Unfortunately we cannot change back classic mode from a claims based web app. So we had to create a new webapplication and copy everything from one site to the other (or create a backup of your sitecollection and restore it in a new web application.