A variety of log destinations are used by Report server and Report server environment to record information about server operations and status. Generally, there are two basic categories of logging, Execution logging and Trace logging. Execution logging basically comprises information about report execution statistics, auditing, performance diagnosis, and optimization. Trace logging comprises information about error messages and general diagnostics.
You can find your log file in this source:
C:\Program Files\Microsoft SQL Server\MSRS10_50.SSRSBI\Reporting Services\LogFiles
MSRS10_50.SSRSBI = It is the name of your SSRS instance Name
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