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/`).
If you cannot even log in because of this error, you can disable security in jenkins config file /config.xml.
Search for `<useSecurity>true</useSecurity>` and change the value to `false`. Then restart jenkins from command line and you should be able to login and make change to plugin/auth configurations as suggested in other answers.