That's the second time I've got this issue when restarting hudson.
This is related to my security configuration as my workaround to get Hudson up
and running again is to edit the config.xml file by hand, replacing the security
configuration with the default one (no security at all), and set it up again
throught the Hudson conf page.
That being done, it works as previously except that if I try to restart Hudson
again, it crashes with the same exception.

Here is how I restart Hudson (correct me if I'm wrong with it) :
"Prepare for Shutdown" from the Manage Hudson page
Stop and Start Tomcat (6.0.18, JDK 6.0u14, running under Fedora)
That's it !

Here is the exception written on the main page when Hudson is up :
hudson.util.IOException2: Unable to read /usr/hudson/config.xml
at hudson.XmlFile.unmarshal(XmlFile.java:150)
at hudson.model.Hudson.load(Hudson.java:1975)
at hudson.model.Hudson.(Hudson.java:557)
at hudson.WebAppMain$2.run(WebAppMain.java:190)
Caused by: com.thoughtworks.xstream.converters.ConversionException: null : null
---- Debugging information ----
cause-exception : java.lang.IllegalArgumentException
cause-message : null
class : hudson.model.Hudson
required-type : hudson.security.GlobalMatrixAuthorizationStrategy
path : /hudson/authorizationStrategy/permission[21]
line number : 28
-------------------------------
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:89)
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.unmarshallField(RobustReflectionConverter.java:262)
at
hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:222)
at
hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:173)
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.core.TreeUnmarshaller.start(TreeUnmarshaller.java:137)
at
com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:33)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:923)
at hudson.util.XStream2.unmarshal(XStream2.java:65)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:909)
at hudson.XmlFile.unmarshal(XmlFile.java:146)
... 3 more
Caused by: java.lang.IllegalArgumentException
at
hudson.security.GlobalMatrixAuthorizationStrategy.add(GlobalMatrixAuthorizationStrategy.java:85)
at
hudson.security.GlobalMatrixAuthorizationStrategy.add(GlobalMatrixAuthorizationStrategy.java:99)
at
hudson.security.GlobalMatrixAuthorizationStrategy.access$200(GlobalMatrixAuthorizationStrategy.java:65)
at
hudson.security.GlobalMatrixAuthorizationStrategy$ConverterImpl.unmarshal(GlobalMatrixAuthorizationStrategy.java:215)
at hudson.util.XStream2$AssociatedConverterImpl.unmarshal(XStream2.java:141)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:82)
... 19 more

Here is my config.xml faulty file :
<?xml version='1.0' encoding='UTF-8'?>
<hudson>
<version>1.319</version>
<numExecutors>2</numExecutors>
<mode>NORMAL</mode>
<useSecurity>true</useSecurity>
<authorizationStrategy class="hudson.security.GlobalMatrixAuthorizationStrategy">
<permission>hudson.model.Computer.Delete:admin</permission>
<permission>hudson.model.View.Configure:admin</permission>
<permission>hudson.scm.SCM.Tag:admin</permission>
<permission>hudson.scm.SCM.Tag:team</permission>
<permission>hudson.model.Hudson.Administer:admin</permission>
<permission>hudson.model.Item.Configure:admin</permission>
<permission>hudson.model.View.Create:admin</permission>
<permission>hudson.model.Run.Delete:admin</permission>
<permission>hudson.model.Item.Create:admin</permission>
<permission>hudson.model.Item.Build:admin</permission>
<permission>hudson.model.Item.Build:team</permission>
<permission>hudson.model.Item.Release:admin</permission>
<permission>hudson.model.Item.Release:team</permission>
<permission>hudson.model.View.Delete:admin</permission>
<permission>hudson.model.Item.Workspace:anonymous</permission>
<permission>hudson.model.Item.Workspace:admin</permission>
<permission>hudson.model.Item.Workspace:team</permission>
<permission>hudson.model.Item.Delete:admin</permission>
<permission>hudson.model.Hudson.Read:anonymous</permission>
<permission>hudson.model.Hudson.Read:admin</permission>
<permission>hudson.model.Hudson.Read:team</permission>
<permission>hudson.model.Item.Read:anonymous</permission>
<permission>hudson.model.Item.Read:admin</permission>
<permission>hudson.model.Item.Read:team</permission>
<permission>hudson.model.Run.Update:admin</permission>
<permission>hudson.model.Computer.Configure:admin</permission>
</authorizationStrategy>
<securityRealm class="hudson.security.HudsonPrivateSecurityRealm">
<disableSignup>true</disableSignup>
</securityRealm>
[...]