Help needed with GAE deployment

I’m having trouble deploying a Vaadin 7 app to GAE. I followed the instructions given
, but when deploying, I get an Error: NOT_FOUND in browser at
If I modify the project’s web.xml settings and map a servlet to the main UI class, I get another error in my browser which is:
500 Server Error: The server encountered an error and could not complete your request.
How can I get my Vaadin 7 project to run on GAE?