We're getting following the errors on the Activiti login screen when I deployed activiti on Websphere.  We tried to add the manger session in context.xml but it didn't fix the issue. 
<?xml version="1.0" encoding="UTF-8"?>
<Context antiJARLocking="true" path="/activiti-explorer">
   <Manager pathname="" />
</Context>
Error 500: java.lang.IllegalArgumentException: Location cannot be null in javax.servlet.http.HttpServletResponse.sendRedirect(location)
WASSession    E DatabaseHashMap serializeAppData SESN0040E: Exception caught while trying to serialize session data for subsequent database write.
WASSessionCor E DatabaseHashMap serializeAppData Exception is: java.io.NotSerializableException: org.activiti.workflow.simple.converter.json.SimpleWorkflowJsonConverter
Any help is greatly appreciated.