Rudy Rodarte explains where you can find SQL Server Reporting Services log files:
After many years working with SQL Server Reporting Services, I’ve gotten myself into a few situations where I had to examine the SSRS log to see what was happening with data sources, subscriptions, and other, unique SSRS particulars. For a time, I thought that the regular SQL Server Error Log was enough. But experience has taught me that there is much more information inside of the SSRS log file than the SQL Server Agent and Service logs. But, you must know where to look to find the SSRS Log file.
Those logs often contain good information.