Feel very frustrated about upgrading.

I followed the instruction tightly. Already changed the machinkey and connection string in development.configuration and release configuration. And successfully triggered the installation, the installation completed with successful notice. Then I click on the link to access the portal. However, I failed, the runtime error page comes out like below:

Server Error in '/' Application.

---

Runtime Error

Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine.

Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off".

I will really appreciate that if someone can help me to solve this problem. Any inputs will be highly appreciated!