Jenkins and Ruby Metric Plugin error

Question

I'm running Jenkins 1.424.2 for a Ruby on Rails app and I want to use the Ruby Metrics plugin. I've installed it using the manage plugins site. But when I restart Jenkins, the plugin is not available at the config options of a job.

So, I had a look into the log files, and it turned out, that the plugin couldn't be started. First some errors with a dependency maven-plugin and then this error

SEVERE: Failed Initializing plugin rubyMetrics
java.lang.NullPointerException

Here is the whole output of the logfile:

...
Running from: /usr/share/jenkins/jenkins.war
[Winstone 2012/02/23 16:53:57] - Beginning extraction from war file
Jenkins home directory: /var/lib/jenkins found at: EnvVars.masterEnvVars.get("JENKINS_HOME")
[Winstone 2012/02/23 16:53:59] - HTTP Listener started: port=8080
[Winstone 2012/02/23 16:53:59] - Winstone Servlet Engine v0.9.10 running: controlPort=disabled
Feb 23, 2012 4:53:59 PM jenkins.model.Jenkins$6 onAttained
INFO: Started initialization
Feb 23, 2012 4:54:00 PM jenkins.model.Jenkins$6 onAttained
INFO: Listed all plugins
Feb 23, 2012 4:54:00 PM jenkins.model.Jenkins$6 onTaskFailed
SEVERE: Failed Loading plugin rubyMetrics
java.io.IOException: Dependency maven-plugin (1.397) doesn't exist
    at hudson.PluginWrapper.resolvePluginDependencies(PluginWrapper.java:424)
    at hudson.PluginManager$2$1$1.run(PluginManager.java:284)
    at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
    at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
    at jenkins.model.Jenkins$5.runTask(Jenkins.java:797)
    at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
    at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:679)
Feb 23, 2012 4:54:00 PM hudson.plugins.ansicolor.PluginImpl start
INFO: starting ansicolor plugin (https://github.com/dblock/jenkins-ansicolor-plugin)
Feb 23, 2012 4:54:00 PM jenkins.model.Jenkins$6 onAttained
INFO: Prepared all plugins
Feb 23, 2012 4:54:00 PM jenkins.model.Jenkins$6 onTaskFailed
SEVERE: Failed Initializing plugin rubyMetrics
java.lang.NullPointerException
    at hudson.PluginManager$2$1$2.run(PluginManager.java:301)
    at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
    at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
    at jenkins.model.Jenkins$5.runTask(Jenkins.java:797)
    at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
    at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:679)
Feb 23, 2012 4:54:00 PM jenkins.model.Jenkins$6 onAttained
INFO: Started all plugins
null
Feb 23, 2012 4:54:00 PM jenkins.model.Jenkins$6 onAttained
INFO: Augmented all extensions
hudson.util.IOException2: Unable to read /var/lib/jenkins/jobs/Raffler/builds/2012-02-23_16-28-13/build.xml
    at hudson.XmlFile.unmarshal(XmlFile.java:155)
    at hudson.model.Run.reload(Run.java:283)
    at hudson.model.Run.<init>(Run.java:272)
    at hudson.model.AbstractBuild.<init>(AbstractBuild.java:159)
    at hudson.model.Build.<init>(Build.java:100)
    at hudson.model.FreeStyleBuild.<init>(FreeStyleBuild.java:41)
    at sun.reflect.GeneratedConstructorAccessor7.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:532)
    at hudson.model.AbstractProject.loadBuild(AbstractProject.java:947)
    at hudson.model.AbstractProject$1.create(AbstractProject.java:256)
    at hudson.model.AbstractProject$1.create(AbstractProject.java:254)
    at hudson.model.RunMap.load(RunMap.java:221)
    at hudson.model.AbstractProject.onLoad(AbstractProject.java:254)
    at hudson.model.Project.onLoad(Project.java:88)
    at hudson.model.Items.load(Items.java:115)
    at jenkins.model.Jenkins$14.run(Jenkins.java:2273)
    at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
    at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
    at jenkins.model.Jenkins$5.runTask(Jenkins.java:797)
    at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
    at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:679)
Caused by: com.thoughtworks.xstream.io.StreamException:  : input contained no data
    at com.thoughtworks.xstream.io.xml.XppReader.pullNextEvent(XppReader.java:80)
    at com.thoughtworks.xstream.io.xml.AbstractPullReader.readRealEvent(AbstractPullReader.java:154)
    at com.thoughtworks.xstream.io.xml.AbstractPullReader.readEvent(AbstractPullReader.java:147)
    at com.thoughtworks.xstream.io.xml.AbstractPullReader.move(AbstractPullReader.java:126)
    at com.thoughtworks.xstream.io.xml.AbstractPullReader.moveDown(AbstractPullReader.java:111)
    at com.thoughtworks.xstream.io.xml.XppReader.<init>(XppReader.java:48)
    at com.thoughtworks.xstream.io.xml.XppDriver.createReader(XppDriver.java:44)
    at com.thoughtworks.xstream.io.xml.XppDriver.createReader(XppDriver.java:49)
    at hudson.XmlFile.unmarshal(XmlFile.java:153)
    ... 24 more
Caused by: java.io.EOFException: input contained no data
    at org.xmlpull.mxp1.MXParser.fillBuf(MXParser.java:3003)
    at org.xmlpull.mxp1.MXParser.more(MXParser.java:3046)
    at org.xmlpull.mxp1.MXParser.parseProlog(MXParser.java:1410)
    at org.xmlpull.mxp1.MXParser.nextImpl(MXParser.java:1395)
    at org.xmlpull.mxp1.MXParser.next(MXParser.java:1093)
    at com.thoughtworks.xstream.io.xml.XppReader.pullNextEvent(XppReader.java:63)
    ... 32 more
Feb 23, 2012 4:54:01 PM jenkins.model.Jenkins$6 onAttained
INFO: Loaded all jobs
Feb 23, 2012 4:54:01 PM jenkins.model.Jenkins$6 onAttained
INFO: Completed initialization
Feb 23, 2012 4:54:01 PM hudson.TcpSlaveAgentListener <init>
INFO: JNLP slave agent listener started on TCP port 52218
Feb 23, 2012 5:00:02 PM hudson.model.Run run
INFO: Raffler #29 main build action completed: SUCCESS
Feb 23, 2012 5:03:19 PM hudson.model.Run run
INFO: Raffler #30 main build action completed: SUCCESS
...

Is this a bug in jenkins or in the plugin? How can i fix it?