I create a Report Book in the Standalone designer that is connected to my Report Server. When I add the first two reports, the Report Book is rendered as expected. When I add the third one, it fails with the exception:

Report processing was cancelled.
Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Here is a link to a short video demonstrating the problem. All the individual reports run correctly.