500 Server Error on Google App Engine - google-app-engine

Question

I am creating some toy REST API using Eclipse + GAE plugin. The App runs great locally, but I got "500 Server Error" when I successfully deployed the code on Google App Engine.
I can access the 'index' page of the App though, but the API part is not functioning..
Error: Server Error
The server encountered an error and could not complete your request.
If the problem persists, please report your problem and mention this error message and the query that caused it.
Here is some extra info from App Engine's log trace:
Uncaught exception from servlet
java.lang.IncompatibleClassChangeError: Implementing class
at com.google.appengine.runtime.Request.process-3ebd0f261ef448d0(Request.java)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
at sun.reflect.GeneratedMethodAccessor5.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
at com.sun.jersey.api.core.ScanningResourceConfig.init(ScanningResourceConfig.java:79)
at com.sun.jersey.api.core.PackagesResourceConfig.init(PackagesResourceConfig.java:104)
at com.sun.jersey.api.core.PackagesResourceConfig.<init>(PackagesResourceConfig.java:78)
at com.sun.jersey.api.core.PackagesResourceConfig.<init>(PackagesResourceConfig.java:89)
at com.sun.jersey.spi.container.servlet.WebComponent.createResourceConfig(WebComponent.java:700)
at com.sun.jersey.spi.container.servlet.WebComponent.createResourceConfig(WebComponent.java:678)
at com.sun.jersey.spi.container.servlet.WebComponent.init(WebComponent.java:203)
at com.sun.jersey.spi.container.servlet.ServletContainer.init(ServletContainer.java:374)
at com.sun.jersey.spi.container.servlet.ServletContainer.init(ServletContainer.java:557)
at javax.servlet.GenericServlet.init(GenericServlet.java:212)
at org.mortbay.jetty.servlet.ServletHolder.initServlet(ServletHolder.java:440)
at org.mortbay.jetty.servlet.ServletHolder.doStart(ServletHolder.java:263)
at org.mortbay.component.AbstractLifeCycle.start(AbstractLifeCycle.java:50)
at org.mortbay.jetty.servlet.ServletHandler.initialize(ServletHandler.java:685)
at org.mortbay.jetty.servlet.Context.startContext(Context.java:140)
at org.mortbay.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1250)
at org.mortbay.jetty.handler.ContextHandler.doStart(ContextHandler.java:517)
at org.mortbay.jetty.webapp.WebAppContext.doStart(WebAppContext.java:467)
at org.mortbay.component.AbstractLifeCycle.start(AbstractLifeCycle.java:50)
at com.google.tracing.TraceContext$TraceContextRunnable.runInContext(TraceContext.java:452)
at com.google.tracing.TraceContext$TraceContextRunnable$1.run(TraceContext.java:458)
at com.google.tracing.TraceContext.runInContext(TraceContext.java:698)
at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContextNoUnref(TraceContext.java:336)
at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContext(TraceContext.java:328)
at com.google.tracing.TraceContext$TraceContextRunnable.run(TraceContext.java:456)
at java.lang.Thread.run(Thread.java:679)
What would be the cause? Any ideas? Thank you everyone!