I've updated plugins and after restart Hudson (ver. 1.394) fails to start up. Log file contains (sorry, I've lost the log and here only what left in my clipboard):

Jan 25, 2011 10:28:45 AM hudson.util.RobustReflectionConverter doUnmarshal
WARNING: Skipping a non-existent field root
com.thoughtworks.xstream.converters.reflection.NonExistentFieldException: No such field hudson.scm.SubversionSCM$SvnInfo.root
at com.thoughtworks.xstream.converters.reflection.FieldDictionary.field(FieldDictionary.java:106)
at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.getFieldType(PureJavaReflectionProvider.java:152)
at hudson.util.RobustReflectionConverter.determineType(RobustReflectionConverter.java:356)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:229)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:180)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:82)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:63)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:76)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:60)
at com.thoughtworks.xstream.converters.collections.AbstractCollectionConverter.readItem(AbstractCollectionConverter.java:71)
at com.thoughtworks.xstream.converters.collections.MapConverter.populateMap(MapConverter.java:75)
at com.thoughtworks.xstream.converters.collections.TreeMapConverter.unmarshal(TreeMapConverter.java:80)
at hudson.util.CopyOnWriteMap$Tree$ConverterImpl.unmarshal(CopyOnWriteMap.java:221)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:82)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:63)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:76)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:60)
at hudson.util.RobustReflectionConverter.unmarshalField(RobustReflectionConverter.java:290)
...
at hudson.XmlFile.read(XmlFile.java:130)
at hudson.model.Queue.load(Queue.java:286)
...
Failed to load the queue file /home/buildbot/.hudson/queue.xml

As I understand the deserialization failed because of changed/missed field in plugin data. All suggestions what to do say to discard unreadable data, but in my case Hudson did not started.

To workaround the problem I've removed /home/buildbot/.hudson/queue.xml, then Huston started and I was able to discard unreadable data.