Try putting a default Index.html page in the WebContent and see if the welcome page points to it. If yes then try entering the servlet name manually and see if that works. I get the same error but doing so fixes it.
http://localhost:8080/<Project_Name>/index.html
http://localhost:8080/<Project_Name>/servlet_name
Open the properties page of your dynamic web project, click on "web project settings" and use the context root paramter as the root path for your URL.