The following happens when the Build-monitor plugin is installed in Jenkins (we where on version 2.10), and then I upgraded to version 2.18.

If the Build-monitor plugin is removed before the upgrade, then re-installed after the upgrade it works fine, so there seems to be something wrong in the Build-monitor plugin.

The attached log-file shows the full trace from the 'jenkins.log' file for ONE short attempt of starting up Jenkins with the plugin enabled.

Jenkins refuse to "startup" after upgrade to 2.18, the log-file is packed with messages like:

Aug 16, 2016 1:19:52 PM hudson.model.RunMap retrieve
WARNING: could not load /home/localjenkins/jobs/Execute-regtest-cmw-4.0-CP/builds/254
java.lang.ClassCastException: java.lang.Integer cannot be cast to hudson.model.Action
at hudson.model.Run.onLoad(Run.java:346)
at hudson.model.RunMap.retrieve(RunMap.java:224)
at hudson.model.RunMap.retrieve(RunMap.java:56)
at jenkins.model.lazy.AbstractLazyLoadRunMap.load(AbstractLazyLoadRunMap.java:500)
at jenkins.model.lazy.AbstractLazyLoadRunMap.load(AbstractLazyLoadRunMap.java:482)
at jenkins.model.lazy.AbstractLazyLoadRunMap.getByNumber(AbstractLazyLoadRunMap.java:380)
at jenkins.model.lazy.AbstractLazyLoadRunMap.search(AbstractLazyLoadRunMap.java:345)
at jenkins.model.lazy.AbstractLazyLoadRunMap.newestBuild(AbstractLazyLoadRunMap.java:275)
at jenkins.model.lazy.LazyBuildMixIn.getLastBuild(LazyBuildMixIn.java:247)
at hudson.model.AbstractProject.getLastBuild(AbstractProject.java:994)
at hudson.model.AbstractProject.getLastBuild(AbstractProject.java:145)
at com.smartcodeltd.jenkinsci.plugins.buildmonitor.viewmodel.JobView.buildViewOf(JobView.java:135)
at com.smartcodeltd.jenkinsci.plugins.buildmonitor.viewmodel.JobView.lastBuild(JobView.java:118)
at com.smartcodeltd.jenkinsci.plugins.buildmonitor.viewmodel.features.KnowsLastBuildDetails.asJson(KnowsLastBuildDetails.java:26)
at com.smartcodeltd.jenkinsci.plugins.buildmonitor.viewmodel.features.KnowsLastBuildDetails.asJson(KnowsLastBuildDetails.java:11)
at com.smartcodeltd.jenkinsci.plugins.buildmonitor.viewmodel.JobViewSerialiser.serialize(JobViewSerialiser.java:25)
at com.smartcodeltd.jenkinsci.plugins.buildmonitor.viewmodel.JobViewSerialiser.serialize(JobViewSerialiser.java:13)
at org.codehaus.jackson.map.ser.std.StdContainerSerializers$IndexedListSerializer.serializeContents(StdContainerSerializers.java:122)
at org.codehaus.jackson.map.ser.std.StdContainerSerializers$IndexedListSerializer.serializeContents(StdContainerSerializers.java:71)
at org.codehaus.jackson.map.ser.std.AsArraySerializerBase.serialize(AsArraySerializerBase.java:86)
at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:446)
at org.codehaus.jackson.map.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:150)
at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:112)
at org.codehaus.jackson.map.ser.StdSerializerProvider._serializeValue(StdSerializerProvider.java:610)
at org.codehaus.jackson.map.ser.StdSerializerProvider.serializeValue(StdSerializerProvider.java:256)
at org.codehaus.jackson.map.ObjectMapper._configAndWriteValue(ObjectMapper.java:2575)
at org.codehaus.jackson.map.ObjectMapper.writeValueAsString(ObjectMapper.java:2097)
at com.smartcodeltd.jenkinsci.plugins.buildmonitor.api.Respond.withSuccess(Respond.java:15)
at com.smartcodeltd.jenkinsci.plugins.buildmonitor.BuildMonitorView.fetchJobViews(BuildMonitorView.java:136)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:324)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:167)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:100)
at org.kohsuke.stapler.MetaClass$JavaScriptProxyMethodDispatcher.doDispatch(MetaClass.java:474)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.MetaClass$11.dispatch(MetaClass.java:380)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:813)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
at org.kohsuke.stapler.Stapler.service(Stapler.java:199)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1669)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:135)
at com.smartcodeltd.jenkinsci.plugin.assetbundler.filters.LessCSS.doFilter(LessCSS.java:47)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at hudson.plugins.audit_trail.AuditTrailFilter.doFilter(AuditTrailFilter.java:95)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:59)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:126)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:49)
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:135)
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$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

And then it continues with:

Aug 16, 2016 1:19:52 PM hudson.model.RunMap retrieve
WARNING: could not load /home/localjenkins/jobs/cmw4-ft-smoke-test/builds/4350
java.lang.ClassCastException

Aug 16, 2016 1:19:52 PM hudson.model.RunMap retrieve
WARNING: could not load /home/localjenkins/jobs/Execute-regtest-cmw-4.0-CP/builds/249
java.lang.ClassCastException

Aug 16, 2016 1:19:52 PM hudson.model.RunMap retrieve
WARNING: could not load /home/localjenkins/jobs/Execute-regtest-cmw-4.0-CP/builds/248
java.lang.ClassCastException

Aug 16, 2016 1:19:52 PM hudson.model.RunMap retrieve
WARNING: could not load /home/localjenkins/jobs/cmw4-ft-smoke-test/builds/4349
java.lang.ClassCastException

Aug 16, 2016 1:19:52 PM hudson.model.RunMap retrieve
WARNING: could not load /home/localjenkins/jobs/Execute-regtest-cmw-4.0-CP/builds/247
java.lang.ClassCastException

Aug 16, 2016 1:19:52 PM hudson.model.RunMap retrieve
WARNING: could not load /home/localjenkins/jobs/Execute-regtest-cmw-4.0-CP/builds/246
java.lang.ClassCastException

Aug 16, 2016 1:19:52 PM hudson.model.RunMap retrieve
WARNING: could not load /home/localjenkins/jobs/cmw4-ft-smoke-test/builds/4348
java.lang.ClassCastException

I've verified the folders (/home/localjenkins/jobs/XYZ-jobname/builds/.....) and they seem ok.