How to get detailed errors from Reporting Services Authentication Extension-Collection of common programming errors

I’m developing Authentication Extension for SQL Server Reporting Services 2008 R2. It works for accessing report manager or report service using browser but trying to open a report in aspx page using ReportViewer control gives following error:

The Authentication Extension threw an unexpected exception or returned a value that is not valid: . (rsAuthenticationExtensionError) Get Online Help For more information about this error navigate to the report server on the local server machine, or enable remote errors

Is is possible to get detailed information of what happened wrong? I run it on local server machine and custom errors are Off in Report Server’s web.config. Nothing is written to Application event log either.