I just ran into this problem with Jenkins version 1.524:
10:53:58 FATAL: hudson/maven/MavenModuleSetBuild
10:53:58 java.lang.NoClassDefFoundError: hudson/maven/MavenModuleSetBuild
10:53:58 at hudson.plugins.copyartifact.CopyArtifact.perform(CopyArtifact.java:221)
10:53:58 at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
10:53:58 at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:804)
10:53:58 at hudson.model.Build$BuildExecution.build(Build.java:199)
10:53:58 at hudson.model.Build$BuildExecution.doRun(Build.java:160)
10:53:58 at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:586)
10:53:58 at hudson.model.Run.execute(Run.java:1593)
10:53:58 at hudson.matrix.MatrixRun.run(MatrixRun.java:146)
10:53:58 at hudson.model.ResourceController.execute(ResourceController.java:88)
10:53:58 at hudson.model.Executor.run(Executor.java:247)
10:53:58 Caused by: java.lang.ClassNotFoundException: hudson.maven.MavenModuleSetBuild
10:53:58 at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
10:53:58 at java.security.AccessController.doPrivileged(Native Method)
10:53:58 at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
10:53:58 at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
10:53:58 at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
10:53:58 at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:334)
10:53:58 ... 10 more
The problem was fixed by enabling the "Maven Integration plugin". I don't have any maven related configurations/jobs in this setup and therefore haven't had any Maven (related) plugins enabled so far, when upgrading from Jenkins 1.522 to 1.524 and also updating all related plugins this broke as noted above.