ERROR: Server Error in /folder Application.
Description: The application attempted to perform an operation not allowed by the security policy. To grant this application the required permission please contact your system administrator or change the application's trust level in the configuration file.
If you find above error while running ASP.net webpages from your browser, it is because your domain is hosted on shared hosting environment and our shared hosting environment only runs at medium trust level.
Users will not have privilege to change the ASP.NET2.0 trust level due to security considerations. As far as security is concern, the full trus level setting is not possible for shared hosting environment to be turned on and it is not possible to overwrite from the client side as well.
However, you can check the following MSDN link for further information and make necessary changes to your script in order to make it running under medium trust level.
You may try always put tag -- trust level="Medium" -- under system.web in web.config when developing an apps to make sure the trust level same as the machine trust level. Thus, you can see the problem in localhost first without testing on the production hosting server.