"A JSONObject text must begin with '{' at character 1" after upgrading to V

I started Jetty server with the app originally created using Vaading 7.0.6 and now using Vaadin 7.3.2 as the dependency.
I see this stacktrace when I click any buttons in the applications (See below).
I guess I missed some 7.0.6->7.3.2 migration step… but then I’d expect the library to fail-fast with a meaningful error message if some pre-configuration is not done… instead, it shows pretty bizzare error.
I see that the JSon string it complains about is:

“6b6e0a51-0329-4cb2-86a5-9cb7062157f5[[“158”,“com.vaadin.shared.ui.ui.UIServerRpc”,“resize”,[“1160”,“1318”,“1318”,“1160”]
],[“233”,“com.vaadin.shared.ui.orderedlayout.AbstractOrderedLayoutServerRpc”,“layoutClick”,[{“relativeY”:“23”, “relativeX”:“394”, “button”:“LEFT”, “clientX”:“773”, “ctrlKey”:false, “metaKey”:false, “clientY”:“435”, “shiftKey”:false, “type”:“8”, “altKey”:false},null]
]]”

Oct 03, 2014 8:29:16 PM com.vaadin.server.communication.UidlRequestHandler synchronizedHandleRequest
SEVERE: Error writing JSON to response
org.json.JSONException: A JSONObject text must begin with '{' at character 1
    at org.json.JSONTokener.syntaxError(JSONTokener.java:496)
    at org.json.JSONObject.<init>(JSONObject.java:180)
    at org.json.JSONObject.<init>(JSONObject.java:403)
    at com.vaadin.server.communication.ServerRpcHandler$RpcRequest.<init>(ServerRpcHandler.java:80)
    at com.vaadin.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:172)
    at com.vaadin.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:93)
    at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
    at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1402)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:305)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
    at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:684)
    at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:503)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
    at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:533)
    at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
    at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086)
    at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:429)
    at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
    at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
    at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:255)
    at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:154)
    at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
    at org.eclipse.jetty.server.Server.handle(Server.java:370)
    at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:494)
    at org.eclipse.jetty.server.AbstractHttpConnection.content(AbstractHttpConnection.java:982)
    at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.content(AbstractHttpConnection.java:1043)
    at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:865)
    at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:240)
    at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
    at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:696)
    at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:53)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
    at java.lang.Thread.run(Thread.java:744)

I just created a sample Spring Boot app and Vaadin and get this error too.

This is added in the pom.xml

<dependency>
  <groupId>org.vaadin.spring</groupId>
  <artifactId>spring-boot-vaadin</artifactId>
  <version>0.0.3-SNAPSHOT</version>
</dependency>
<dependency>
   <groupId>com.vaadin</groupId>
   <artifactId>vaadin-themes</artifactId>
   <version>7.1.13</version>
</dependency>
<dependency>
   <groupId>com.vaadin</groupId>
   <artifactId>vaadin-client-compiled</artifactId>
   <version>7.1.13</version>
</dependency>
@VaadinUI
@Title("Addressbook")
public class MyUI extends UI {

     @Override
        protected void init(VaadinRequest vaadinRequest) {
         getPage().setTitle("Vaadin4Spring Side Bar Sample");
            final HorizontalSplitPanel rootLayout = new HorizontalSplitPanel();
            rootLayout.setStyleName(Reindeer.SPLITPANEL_SMALL);
            rootLayout.setSizeFull();
            setContent(rootLayout);
        }
} 

This runs fine and I see a splitpanel. But when trying to size the splitter I get this error too.

2014-10-04 14:50:57.607  WARN 6032 --- [io-8080-exec-10]
 c.v.s.communication.UidlRequestHandler   : 
=================================================================
The widgetset in use does not seem to be built for the Vaadin
version in use. This might cause strange problems - a
recompile/deploy is strongly recommended.
 Vaadin version: 7.3.1
 Widgetset version: 7.1.13
=================================================================
2014-10-04 14:50:57.608 ERROR 6032 --- [io-8080-exec-10]
 c.v.s.communication.UidlRequestHandler   : Error writing JSON to response

org.json.JSONException: A JSONObject text must begin with '{' at character 1
    at org.json.JSONTokener.syntaxError(JSONTokener.java:496)
    at org.json.JSONObject.<init>(JSONObject.java:180)
    at org.json.JSONObject.<init>(JSONObject.java:403)
    at com.vaadin.server.communication.ServerRpcHandler$RpcRequest.<init>(ServerRpcHandler.java:80)

Well, that warning pretty much explains it. You’re using vaadin-themes and vaadin-client-compiled version 7.1.3 even though your serverside is 7.3.1.

Aah, it’s working now!
Changed to:

<dependency>
  <groupId>org.vaadin.spring</groupId>
  <artifactId>spring-boot-vaadin</artifactId>
  <version>0.0.3-SNAPSHOT</version>
</dependency>
<dependency>
   <groupId>com.vaadin</groupId>
   <artifactId>vaadin-themes</artifactId>
   <version>7.3.1</version>
</dependency>
<dependency> 
   <groupId>com.vaadin</groupId>
   <artifactId>vaadin-client-compiled</artifactId>
   <version>7.3.1</version>
</dependency>