My Environment:
1. SSRS 2005 64bit
2. Windows 2008 R2 64bit
3. IIS 7.5
4. After completing the Reporting services set-up, I have got all necessary green mark 
on the configuration manager window.
Then I tried to access the Reports url http://localhost/reports and I have got the following error message!
And the Source Error and Stack trace are:
To fix this issue, just follow these steps, the same modification of the “RSWebApplication.config” will solve this issue too.
Step Bi Step 