I'm currently facing a problem using Spring Web Flows 2.0.7, the webflow crashes, when a variable is submitted. The corresponding webflow looks like this:

PHP Code:
    <flow xmlns="http://www.springframework.org/schema/webflow"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://www.springframework.org/schema/webflow
                              http://www.springframework.org/schema/webflow/spring-webflow-2.0.xsd">
    
      <!-- some model, i use below -->
      <var name="register"
           class="org.zcore.conference.Registration" />
    
      <!-- my input id -->
      <input type="long" name="conId" value="flowScope.conId"/>
    
      <!-- my first view state -->
      <view-state id="register" model="register">
      <!-- imagine much more stuff here -->
    </flow> 
When entering the flow with an appended conId (http://localhost:8080/app/myflow?conId=123) the following error pops up:

PHP Code:
    2012-08-07 17:33:34,583 WARN 25205715@qtp-28882952-5 com.coremedia.cae.webflow.AbstractFlowViewController - Error handling flow request: 'Could not serialize flow execution; make sure all objects stored in flow or flash scope are serializable': /app/flow
    org.springframework.webflow.execution.repository.snapshot.SnapshotCreationException: Could not serialize flow execution; make sure all objects stored in flow or flash scope are serializable 
I'm pretty surprised, that the supplied value does not seem to be serializable, as it is a simple int value. Looks like I'm missing something basic here.