again I am seeing this:

May 17, 2013 10:37:03 AM jenkins.InitReactorRunner$1 onTaskFailed
SEVERE: Failed Loading job XXX
hudson.util.IOException2: Unable to read /var/lib/jenkins/jobs/XXX/config.xml
at hudson.XmlFile.read(XmlFile.java:147)
at hudson.model.Items.load(Items.java:220)
at jenkins.model.Jenkins$17.run(Jenkins.java:2554)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:888)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:722)
Caused by: com.thoughtworks.xstream.converters.ConversionException: Invalid reference
---- Debugging information ----
reference : ../../filePathFactory
class : org.jenkinsci.plugins.ivytrigger.util.PropertiesFileContentExtractor
required-type : org.jenkinsci.plugins.ivytrigger.util.PropertiesFileContentExtractor
converter-type : hudson.util.RobustReflectionConverter
path : /project/triggers/org.jenkinsci.plugins.ivytrigger.IvyTrigger/propertiesFileContentExtractor/filePathFactory
line number : 71
class[1] : org.jenkinsci.plugins.ivytrigger.IvyTrigger
class[2] : java.util.Vector
converter-type[1] : hudson.util.RobustCollectionConverter
class[3] : hudson.model.FreeStyleProject
version : null
-------------------------------
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:57)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at hudson.util.RobustReflectionConverter.unmarshalField(RobustReflectionConverter.java:333)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:275)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:222)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at hudson.util.RobustReflectionConverter.unmarshalField(RobustReflectionConverter.java:333)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:275)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:222)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:50)
at com.thoughtworks.xstream.converters.collections.AbstractCollectionConverter.readItem(AbstractCollectionConverter.java:71)
at hudson.util.RobustCollectionConverter.populateCollection(RobustCollectionConverter.java:85)
at com.thoughtworks.xstream.converters.collections.CollectionConverter.unmarshal(CollectionConverter.java:61)
at hudson.util.RobustCollectionConverter.unmarshal(RobustCollectionConverter.java:76)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at hudson.util.RobustReflectionConverter.unmarshalField(RobustReflectionConverter.java:333)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:275)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:222)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:50)
at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:134)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:32)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1061)
at hudson.util.XStream2.unmarshal(XStream2.java:109)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1045)
at com.thoughtworks.xstream.XStream.fromXML(XStream.java:925)
at hudson.XmlFile.read(XmlFile.java:143)
... 10 more

reverted back to 0.24 which is working.

looking in the config.xml of that job is see:

<org.jenkinsci.plugins.ivytrigger.IvyTrigger plugin="ivytrigger@0.24">
<spec># every 10 minutes
H/10 * * * *</spec>
<ivyPath>ivy.xml</ivyPath>
<ivySettingsPath>$SOFCOMMON/conf/ivysettings.xml</ivySettingsPath>
<propertiesFilePath>$SOFCOMMON/conf/ivysettings.properties</propertiesFilePath>
<debug>false</debug>
<filePathFactory/>
<propertiesFileContentExtractor>
<filePathFactory reference="../../filePathFactory"/>
</propertiesFileContentExtractor>
</org.jenkinsci.plugins.ivytrigger.IvyTrigger>

Code changed in jenkins
User: Gregory Boissinot
Path:
src/main/java/org/jenkinsci/plugins/ivytrigger/IvyTrigger.java
http://jenkins-ci.org/commit/ivytrigger-plugin/c5f58b98718b4bab5a75b3d20a1708e244570c87
Log:
Fix JENKINS-17831

Please test again with 0.28

0.28 seems to do the trick. Happy days