App Engine SDK - Google Engine

com.google.apphosting.utils.config.AppEngineConfigException: Invalid configuration

include the following chunk in your appengine-web.xml:

<threadsafe>true</threadsafe>

It is mandatory to explicitly set expectations whether your application is designed in a thread-safe way.

Source:
http://stackoverflow.com/questions/13880528/running-appengine-with-eclipse