First off, I want to start by saying I don't know a whole lot about Reporting Services. 
We recently had an issue where a staff member told us that they hadn't been receiving a certain report. When I logged onto Reporting Services and tested the connection to the data source I was getting "Unspecified Error". Not too helpful. I then tried to find the last successful run. I could not find this in the Reporting Services interface. However, there is a view named ExecutionLog2 that had what I wanted.
SELECT TOP 100 elog.status, elog.TimeStart, elog.*
FROM [dbo].[ExecutionLog2] elog
WHERE elog.reportPath = 'MyReportPath'
AND elog.UserName = 'MyReportUser'
ORDER BY elog.timestart DESC
We actually ended up restarting the Reporting Services service and the "Unspecified Error" went away. I just thought I'd post this in case it helps anyone else.