java.lang.IllegalStateException: getAttribute: Session already invalidated when trying to call getAvailableActions or getFieldsForAction

I wrote a very simple SOAP client to test the available workflow actions on our system (attached). Unfortunately it sporadically crashes when calling getAvailableActions() or getFieldsForAction(), with the attached exceptions. Note that the getIssue call always seems to succeed (both before and after the crash) and I am also able to successfully add comments to actions. I have set up all the SOAP bindings to be in the package com.atlassian.jira.rpc.soapclient.