Similar to the SQL Server Database Engine, SSRS writes detailed trace and error information, useful for troubleshooting, to a log file, namely:
%PROGRAMFILES%\Microsoft SQL Server\MSRS11.[InstanceName]\Reporting Services\LogFiles\ReportServerService_[timestamp].log
The log file contains three types of events: errors, warnings, and informational messages. Each entry begins with a prefix that includes the current time stamp, the process and thread used, the type of event, and the message itself. Note that SSRS automatically deletes trace files older than a certain number of days.