Error on orbeon.log

Where to start debug for this error?
Orbeon version orbeon-3.9.1.201202031753-PE
We don't use Form runner

First error lines here and full error in file:
2013-06-07 13:12:12,203 ERROR ProcessorService  - Exception at line 55, column 65 of oxf:/ops/xforms/xforms-server.xpl (reading processor output: name='response', id='xforms-response')
java.lang.NullPointerException
        at org.orbeon.oxf.xforms.state.XFormsStateManager.checkUUIDInSession(XFormsStateManager.java:92)
        at org.orbeon.oxf.xforms.state.XFormsStateManager.findOrRestoreDocument(XFormsStateManager.java:277)
        at org.orbeon.oxf.xforms.processor.XFormsServer.doIt(XFormsServer.java:152)
...

error.txt

Hi Cass,

This looks like a bug. I see you are on 3.9.1. Would you be able to test this with 4.x? And if you are also getting this with 4.x, would you be able to share with us the steps on how to reproduce it?

Alex