SSRS 2014 - query running fine on management studio - report timing out - how to troubleshoot?
You need to look at the Execution logs. On your Report Server database, there's a view called ExecutionLog3. You can query this view by date and find how the user was running the report.

USE [ReportServer]
GO

SELECT *
FROM dbo.ExecutionLog3 AS el
WHERE el.TimeStart &gt;= '2016-08-02'