I hate to cross post, but I wasn\'t getting any progress with this on serverfault and there appears to be a lot more activity with regard to reporting services here.
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You also can check the Report History, to get this option, put the mouse on top of your report and select manage option from drop-down list of your report, look for the "Report History" on the left options, create a "New Snapshot"
It will show you, the last error know:
Parameter validation failed. It is not possible to provide valid values for all parameters (rsParameterError)
for this case you need to check the definition of the report, I had an issue with the drop-down list I used to send parameters to the report.
You need to look in the "Report Server Execution Log" which is a particular SQL Reporting Services log file.
See Reporting Services Log Files for details.
A bit late to the party, however: Here is where I find my logs when DD subscription fails:
C:\Program Files\Microsoft SQL Server\MSSQL.2\Reporting Services\LogFiles look for filename: ReportServerService*.log with relevant Date Modified
The path under which you can find Report Server Service logs could be different, such as: C:\Program Files\Microsoft SQL Server\MSRS12.MSSQLSERVER\Reporting Services\LogFiles
In this logs you can find errors that happened after generating the report itself. For instance, if the Reporting Services user needs to write the report as a CSV file after a subscription and it lacks the necessary authorization, the only place where you can find the description of the error is in these logs, while the history of the Job connected with the subscription and the Execution Log will tell you that the Report was correctly generated (nothing about saving the report).