jenkins.log when trying to start up Jenkins:

SEVERE: Failed Loading plugin nodeofflinenotification
hudson.util.IOException2: Failed to initialize
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:349)
at hudson.PluginManager$2$1$1.run(PluginManager.java:316)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:878)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
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:636)
Caused by: org.jruby.exceptions.RaiseException: (LoadError) library `fiber' could not be loaded: java.lang.ClassNotFoundException: org.jruby.ext.fiber.ThreadFiberLibrary

Removing the nodeofflinenotification plugin brings up Jenkins again.

PS: I couldn't assign the "nodeofflinenotification" component in the bug report even though it's the one referred to at https://wiki.jenkins-ci.org/display/JENKINS/Node+Offline+Notification+Plugin