When I open Periodic Backup Manager plugin, I see the following stack trace instead of a list of backups:

javax.servlet.ServletException: org.apache.commons.jelly.JellyTagException: jar:file:/var/lib/jenkins/plugins/periodicbackup/WEB-INF/lib/classes.jar!/org/jenkinsci/plugins/periodicbackup/PeriodicBackupLink/index.jelly:47:106: <j:forEach> org.jenkinsci.plugins.periodicjenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_backup.BackupObject at org.kohsuke.stapler.jelly.JellyClassTearOff.serveIndexJelly(JellyClassTearOff.java:117) at org.kohsuke.stapler.jelly.JellyFacet.handleIndexRequest(JellyFacet.java:127) at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:735) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876) at org.kohsuke.stapler.MetaClass$11.dispatch(MetaClass.java:380) at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649) at org.kohsuke.stapler.Stapler.service(Stapler.java:238) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1669) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:135) at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:126) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:86) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84) at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249) at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76) at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143) at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:553) at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223) at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127) at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515) at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185) at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97) at org.eclipse.jetty.server.Server.handle(Server.java:499) at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311) at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257) at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544) at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) at java.lang.Thread.run(Thread.java:745) Caused by: org.apache.commons.jelly.JellyTagException: jar:file:/var/lib/jenkins/plugins/periodicbackup/WEB-INF/lib/classes.jar!/org/jenkinsci/plugins/periodicbackup/PeriodicBackupLink/index.jelly:47:106: <j:forEach> org.jenkinsci.plugins.periodicjenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_backup.BackupObject at org.apache.commons.jelly.impl.TagScript.handleException(TagScript.java:726) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:281) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.ForEachTag.doTag(ForEachTag.java:150) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.OtherwiseTag.doTag(OtherwiseTag.java:41) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.ChooseTag.doTag(ChooseTag.java:38) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$1.run(CoreTagLibrary.java:98) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.JellyViewScript.run(JellyViewScript.java:95) at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:63) at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:53) at org.kohsuke.stapler.jelly.JellyClassTearOff.serveIndexJelly(JellyClassTearOff.java:112) ... 59 more Caused by: com.thoughtworks.xstream.mapper.CannotResolveClassException: org.jenkinsci.plugins.periodicjenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_jenkins_backup.BackupObject at com.thoughtworks.xstream.mapper.DefaultMapper.realClass(DefaultMapper.java:79) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.DynamicProxyMapper.realClass(DynamicProxyMapper.java:55) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.PackageAliasingMapper.realClass(PackageAliasingMapper.java:88) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.ClassAliasingMapper.realClass(ClassAliasingMapper.java:79) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.ArrayMapper.realClass(ArrayMapper.java:74) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.SecurityMapper.realClass(SecurityMapper.java:71) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at hudson.util.XStream2$CompatibilityMapper.realClass(XStream2.java:282) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at org.jenkinsci.jruby.JRubyMapper.realClass(JRubyMapper.java:34) at hudson.util.xstream.MapperDelegate.realClass(MapperDelegate.java:43) at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30) at com.thoughtworks.xstream.mapper.CachingMapper.realClass(CachingMapper.java:48) at com.thoughtworks.xstream.core.util.HierarchicalStreams.readClassType(HierarchicalStreams.java:29) at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:133) at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:32) at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1189) at hudson.util.XStream2.unmarshal(XStream2.java:114) at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1173) at com.thoughtworks.xstream.XStream.fromXML(XStream.java:1044) at com.thoughtworks.xstream.XStream.fromXML(XStream.java:1035) at org.jenkinsci.plugins.periodicbackup.BackupObject$1.apply(BackupObject.java:78) at org.jenkinsci.plugins.periodicbackup.BackupObject$1.apply(BackupObject.java:74) at com.google.common.collect.Iterators$8.next(Iterators.java:812) at com.google.common.collect.Lists.newArrayList(Lists.java:139) at com.google.common.collect.Lists.newArrayList(Lists.java:119) at org.jenkinsci.plugins.periodicbackup.BackupExecutor.backup(BackupExecutor.java:92) at org.jenkinsci.plugins.periodicbackup.PeriodicBackup.execute(PeriodicBackup.java:64) at hudson.model.AsyncPeriodicWork$1.run(AsyncPeriodicWork.java:100) ... 1 more