When an event is defined with update-field set, The event translator cannot clone the event. For example - an event is defined

<event>
<uei>uei.opennms.org/application/mobilelocation/single-working</uei>
...
<alarm-data reduction-key="%uei%:%parm[workerid]%" alarm-type="1" auto-clean="false" >
<update-field field-name="severity" update-on-reduction="true"/>
<update-field field-name="logmsg" update-on-reduction="true"/>
</alarm-data>
</event>

and an event translator translation spec is defined

<event-translation-spec uei="uei.opennms.org/application/mobilelocation/single-working" >
<mappings>
<mapping>
<assignment name="uei" type="field" >
<value type="constant" result="uei.opennms.org/application/mobilelocation/single-working-node-updated" />
</assignment>
</mapping>
</mappings>
</event-translation-spec>

The event translator log shows the following the error;
2014-10-16 14:29:04,303 ERROR [event-translator-Thread] o.o.n.c.EventTranslatorConfigFactory: Exception cloning event
java.io.NotSerializableException: org.opennms.netmgt.xml.event.UpdateField

When the following definitions are removed, the event translator works fine

<update-field field-name="severity" update-on-reduction="true"/>
<update-field field-name="logmsg" update-on-reduction="true"/>

It looks like the org.opennms.netmgt.xml.event.UpdateField is not serializable when the translator tries to clone the event

Full log listing from when event arrives below