Since i upgraded from 1.617 to 1.634 i got EMs in the log, complaining about a faulty dependency between nodeofflinenotification (aka mail watcher plugin) and ruby-runtime:
{{SEVERE: Failed Loading plugin ruby-runtime:

{{SEVERE: Failed Loading plugin ruby-runtime
java.io.IOException: Failed to initialize
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:441)
at hudson.PluginManager$2$1$1.run(PluginManager.java:384)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
at jenkins.model.Jenkins$8.runTask(Jenkins.java:915)
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(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassCircularityError: org/jruby/RubyClass
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at org.jenkinsci.bytecode.ClassWriter.loadClass(ClassWriter.java:97)
at org.jenkinsci.bytecode.ClassWriter.getCommonSuperClass(ClassWriter.java:64)
at org.kohsuke.asm5.ClassWriter.getMergedType(ClassWriter.java:1654)
at org.kohsuke.asm5.Frame.merge(Frame.java:1426)
at org.kohsuke.asm5.Frame.merge(Frame.java:1374)
at org.kohsuke.asm5.MethodWriter.visitMaxs(MethodWriter.java:1475)
at org.kohsuke.asm5.tree.MethodNode.accept(MethodNode.java:833)
at org.kohsuke.asm5.commons.JSRInlinerAdapter.visitEnd(JSRInlinerAdapter.java:187)
at org.jenkinsci.bytecode.Transformer$1$1.visitEnd(Transformer.java:107)
at org.kohsuke.asm5.MethodVisitor.visitEnd(MethodVisitor.java:877)
at org.kohsuke.asm5.ClassReader.readMethod(ClassReader.java:1021)
at org.kohsuke.asm5.ClassReader.accept(ClassReader.java:693)
at org.kohsuke.asm5.ClassReader.accept(ClassReader.java:506)
at org.jenkinsci.bytecode.Transformer.transform(Transformer.java:113)
at hudson.ClassicPluginStrategy$AntClassLoader2.defineClassFromData(ClassicPluginStrategy.java:800)
at jenkins.util.AntClassLoader.getClassFromStream(AntClassLoader.java:1310)
at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1366)
at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1326)
at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1079)
at java.lang.ClassLoader.loadClass(Unknown Source)
at org.jenkinsci.jruby.RubyClassConverter.<init>(RubyClassConverter.java:12)
at org.jenkinsci.jruby.JRubyXStream.register(JRubyXStream.java:25)
at ruby.RubyRuntimePlugin.initRubyXStreams(RubyRuntimePlugin.java:44)
at ruby.RubyRuntimePlugin.start(RubyRuntimePlugin.java:28)
at hudson.ClassicPluginStrategy.startPlugin(ClassicPluginStrategy.java:449)
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:438)
... 9 more

Oct 20, 2015 6:55:13 AM jenkins.InitReactorRunner$1 onTaskFailed
SEVERE: Failed Loading plugin nodeofflinenotification
java.io.IOException: Dependency ruby-runtime (0.9) doesn't exist
at hudson.PluginWrapper.resolvePluginDependencies(PluginWrapper.java:533)
at hudson.PluginManager$2$1$1.run(PluginManager.java:383)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
at jenkins.model.Jenkins$8.runTask(Jenkins.java:915)
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(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
}}