AX infolog error:
An internal error occurred on the report server. See the error log for more details.
Throwing Microsoft.ReportingServices.Diagnostics.Utilities.InternalCatalogException: ScalableList: Index 6 outside the allowed range [0::5]
Event Viewer error:
Internal error: Microsoft.ReportingServices.Diagnostics.Utilities.InternalCatalogException: An internal error occurred on the report server. See the error log for more details.
SSRS logs location: C:\Program Files\Microsoft SQL Server\MSRS13.MSSQLSERVER\Reporting Services\LogFiles
Error appears when report is automatically converted to PDF format (or any other paged format, HTML for example will work correctly). If report is printed on the screen and then converted to a PDF it will finish without errors, but PDF will contains error and it is not possible to open it.
The reason is hidden columns in the tablix. Make them visible or delete. Mentioned index in the SSRS error log usually references the problematic column.