This error might occur due to two scenarios.
1. There is an error in the application's logic with the inputformat, Type etc., and you have set the Custom Error Mode in the web.config to "On" and not specified a default redirect error page.
2. The web.config file is not well formed or having invalid characters and the application is not able to pick up the settings from the same.
1. Set the custom error mode to "Off" to view the error. After rectifying it and before deployment, change it to "On" and specify a default error page, as follows:-
< customErrors defaultRedirect="ErrorPage.aspx" mode="On">
such that your users will not be able to see the actual error and get your friendly error page where you can politely say "An error has occured! Sorry for the inconvenience ..." .
2. If the above solution is not working (i.e. even after setting the custom error mode to On, the same "Server Error" occurs, then the likely chance is that your web.config file is not well formed and has invalid characters etc.,
To resolve, it copy paste the contents of the file to a notepad, save it as an xml file and try to browse the xml file in the browser. If the xml file is unable to be rendered by the browser and throws error, then you can find the place where the tags are not well formed or invalid character(s) exist and rectify them.
Things worth noting is Web.config is case sensitive and even trailing / leading spaces can cause the above error.