Question: I was trying to restore jenkins on a new machine by tacking up backup from old machine . I replaced the jenkins home directory of new machine from old one. When i launch jenkins it gives me this error.

```
Caused: java.io.IOException: Unable to read /var/lib/jenkins/config.xml

```

There is also 

```
 Caused: hudson.util.HudsonFailedToLoad
Caused: org.jvnet.hudson.reactor.ReactorException

```

Debug info is 
---- Debugging information ----

```
message             : hudson.security.ProjectMatrixAuthorizationStrategy
cause-exception     : com.thoughtworks.xstream.mapper.CannotResolveClassException
cause-message       : hudson.security.ProjectMatrixAuthorizationStrategy
class               : hudson.model.Hudson
required-type       : hudson.model.Hudson
converter-type      : hudson.util.RobustReflectionConverter
path                : /hudson/authorizationStrategy
line number         : 11
version             : not available
-------------------------------

```

This is what my config.xml look like 

```
<useSecurity>true</useSecurity>
  <authorizationStrategy class="hudson.security.ProjectMatrixAuthorizationStrategy">
    <permission>hudson.model.Hudson.Administer:visha</permission>
  </authorizationStrategy>

```

Can someone please help ?

Answer: 
This usually happens when the plugin providing the authorization strategy is not installed or enabled.

Make sure the `matrix-auth` plugin is installed and that it's not disabled (no `matrix-auth.jpi.disabled` file (or similar) in `$JENKINS_HOME/plugins/`).
usually, this error ours when there is a mismatch between Jenkins Version and Plugins version.

Best solution is always to keep updated Jenkins and plugin or installed appropriate versions of jenkins according to the Jenkins version.

For centos, Redhat, amazon Linux follow the below steps.

```
sudo rpm --import https://pkg.jenkins.io/redhat-stable/jenkins.io.key
yum update Jenkins. 

```

For an ubuntu machine, you can follow the steps given by hit3k.

```
cd /tmp; wget https://updates.jenkins-ci.org/latest/jenkins.war
cp /usr/share/jenkins/jenkins.war /tmp/jenkins.war.previous.version
sudo systemctl status jenkins
sudo cp /tmp/jenkins.war /usr/share/jenkins/
sudo systemctl restart jenkins

```