Jenkins doesn't work with java.lang.NoClassDefFoundError: jenkins / tasks / SimpleBuildStep

After the build operation completes, the job will exit with the following error. We have installed the naginator plugin and removed it. From this moment it happens. I tried to reinstall the plugin, even then it happens.

FATAL: jenkins/tasks/SimpleBuildStep
java.lang.NoClassDefFoundError: jenkins/tasks/SimpleBuildStep
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
    at jenkins.util.AntClassLoader.defineClassFromData(AntClassLoader.java:1138)
    at hudson.ClassicPluginStrategy$AntClassLoader2.defineClassFromData(ClassicPluginStrategy.java:755)
    at jenkins.util.AntClassLoader.getClassFromStream(AntClassLoader.java:1309)
    at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1365)
    at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1325)
    at sun.reflect.GeneratedMethodAccessor13.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at jenkins.ClassLoaderReflectionToolkit.invoke(ClassLoaderReflectionToolkit.java:44)
    at jenkins.ClassLoaderReflectionToolkit._findClass(ClassLoaderReflectionToolkit.java:86)
    at hudson.ClassicPluginStrategy$DependencyClassLoader.findClass(ClassicPluginStrategy.java:632)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    at jenkins.util.AntClassLoader.findBaseClass(AntClassLoader.java:1397)
    at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1074)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:191)
    at hudson.plugins.git.GitChangeSet.hasHudsonTasksMailer(GitChangeSet.java:296)
    at hudson.plugins.git.GitChangeSet.findOrCreateUser(GitChangeSet.java:274)
    at hudson.plugins.git.GitChangeSet.getAuthor(GitChangeSet.java:329)
    at hudson.model.AbstractBuild.getCulprits(AbstractBuild.java:352)
    at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:689)
    at hudson.model.Run.execute(Run.java:1757)
    at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
    at hudson.model.ResourceController.execute(ResourceController.java:88)
    at hudson.model.Executor.run(Executor.java:234)
Caused by: java.lang.ClassNotFoundException: jenkins.tasks.SimpleBuildStep
    at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1375)
    at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1325)
    at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1078)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    ... 29 more