This might also be the cause of these issues:

• https://issues.jenkins-ci.org/browse/JENKINS-19245
• https://issues.jenkins-ci.org/browse/JENKINS-18628
• https://issues.jenkins-ci.org/browse/JENKINS-18816

Stack trace:

hudson.util.IOException2: Failed to dynamically deploy this plugin
at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1317)
at hudson.model.UpdateCenter$DownloadJob.run(UpdateCenter.java:1116)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at hudson.remoting.AtmostOneThreadExecutor$Worker.run(AtmostOneThreadExecutor.java:104)
at java.lang.Thread.run(Thread.java:724)
Caused by: hudson.util.IOException2: Failed to install ruby-runtime plugin
at hudson.PluginManager.dynamicLoad(PluginManager.java:438)
at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1313)
... 6 more
Caused by: hudson.util.IOException2: Failed to initialize
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:370)
at hudson.PluginManager.dynamicLoad(PluginManager.java:429)
... 7 more
Caused by: java.lang.NullPointerException
at java.lang.Class.isAssignableFrom(Native Method)
at org.jenkinsci.jruby.JavaProxyConverter.canConvert(JavaProxyConverter.java:43)
at com.thoughtworks.xstream.core.DefaultConverterLookup.registerConverter(DefaultConverterLookup.java:72)
at com.thoughtworks.xstream.XStream.registerConverter(XStream.java:1294)
at com.thoughtworks.xstream.XStream.registerConverter(XStream.java:1289)
at org.jenkinsci.jruby.JRubyXStream.register(JRubyXStream.java:26)
at ruby.RubyRuntimePlugin.initRubyXStreams(RubyRuntimePlugin.java:46)
at ruby.RubyRuntimePlugin.start(RubyRuntimePlugin.java:28)
at hudson.ClassicPluginStrategy.startPlugin(ClassicPluginStrategy.java:378)
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:367)
... 8 more