This error usually means that the ASPNet user does not have adequate permission to access the web.config file. By default, the ASPNet user should have enough permission to access your directory. However, if you create or copy a new application under the root with Visual Studio .NET, the ASPNet user permission may not inherit into the subdirectory automatically.
You can check permission for each folders on your Website in file Manager. Make sure that all the files or folders under public_html has ASP.NET,IUSR and IWAM Permission.
If you still can't fixed the problems,Please login to your Control Panel and create a support ticket.
