title: Unable to retrieve XForms engine state
body: <p>I working with Orbeon XForms 4.7 on eXist-db 2.2. We've always had session problems where a session would expire in eXist-db after a given point in time, but where Orbeon retains a session token and 'thinks' it is still logged on. This may or may not be at the heart of what I see in the logging:</p>
  
  <pre><code>2016-11-07 15:06:27,643 INFO  ProcessorService  - /xforms-server - Received request
  2016-11-07 15:06:27,650 INFO  ProcessorService  - /xforms-server - Timing: 7
  2016-11-07 15:06:29,368 INFO  ProcessorService  - /xforms-server - Received request
  2016-11-07 15:06:29,372 ERROR XFormsServer  - Unable to retrieve XForms engine state. Please reload the current page. Note that you will lose any unsaved changes. {}
  2016-11-07 15:06:29,372 INFO  ProcessorService  - /xforms-server - Timing: 4
  2016-11-07 15:06:29,373 ERROR ProcessorService  - 
  +----------------------------------------------------------------------------------------------------------------------+
  |An Error has Occurred                                                                                                 |
  |----------------------------------------------------------------------------------------------------------------------|
  |Unable to retrieve XForms engine state. Please reload the current page. Note that you will lose any unsaved changes.  |
  |----------------------------------------------------------------------------------------------------------------------|
  |Application Call Stack                                                                                                |
  |----------------------------------------------------------------------------------------------------------------------|
  |oxf:/config/prologue-servlet.xpl                                                  |executing processor           |  36|
  |······················································································································|
  |element=&lt;p:processor name="oxf:pipeline"&gt;[...]&lt;/p:processor&gt;                                                          |
  |name   ={http://www.orbeon.com/oxf/processors}pipeline                                                                |
  |----------------------------------------------------------------------------------------------------------------------|
  |oxf:/ops/xforms/xforms-server.xpl                                                 |reading processor output      |  43|
  |······················································································································|
  |element=&lt;p:output name="response" id="xforms-response"/&gt;                                                              |
  |name   =response                                                                                                      |
  |id     =xforms-response                                                                                               |
  |----------------------------------------------------------------------------------------------------------------------|
  |----------------------------------------------------------------------------------------------------------------------|
  |Exception: org.orbeon.oxf.common.OXFException                                                                         |
  |----------------------------------------------------------------------------------------------------------------------|
  |org.orbeon.oxf.xforms.state.XFormsStateManager     |createDocumentFromStore       |XFormsStateManager.java       | 489|
  |org.orbeon.oxf.xforms.state.XFormsStateManager     |findOrRestoreDocument         |XFormsStateManager.java       | 448|
  |org.orbeon.oxf.xforms.state.XFormsStateManager     |beforeUpdate                  |XFormsStateManager.java       | 328|
  |org.orbeon.oxf.xforms.processor.XFormsServer       |doIt                          |XFormsServer.java             | 169|
  |org.orbeon.oxf.xforms.processor.XFormsServer       |access$000                    |XFormsServer.java             |  59|
  |org.orbeon.oxf.xforms.processor.XFormsServer$1     |readImpl                      |XFormsServer.java             |  85|
  |essor.impl.ProcessorOutputImpl$TopLevelOutputFilter|read                          |ProcessorOutputImpl.java      | 257|
  |org.orbeon.oxf.processor.impl.ProcessorOutputImpl  |read                          |ProcessorOutputImpl.java      | 394|
  |----------------------------------------------------------------------------------------------------------------------|
  |Exception: org.orbeon.oxf.common.ValidationException                                                                  |
  |----------------------------------------------------------------------------------------------------------------------|
  |org.orbeon.oxf.common.OrbeonLocationException$     |wrapException                 |OrbeonLocationException.scala |  60|
  |org.orbeon.oxf.common.OrbeonLocationException      |wrapException                 |OrbeonLocationException.scala |    |
  </code></pre>
  
  <p>This error comes up every such and so seconds. There is no mention what XForms triggers it, so a targeted search is out of the question.</p>
  
  <p>Does anyone know how to interpret this error and possibly how to prevent it? Note that there's no error at the user side at all, and everything appears to work fine. This is purely server side.</p>