After upgrading Jenkins core from v2.208 to v2.209 I see the following things that make me a bit nervous:

• Start-up log

  Running from: /usr/share/jenkins/jenkins.war
  2019-12-17 05:44:39.660+0000 [id=1]     INFO    org.eclipse.jetty.util.log.Log#initialized: Logging initialized @373ms to org.eclipse.jetty.util.log.JavaUtilLog
  2019-12-17 05:44:39.761+0000 [id=1]     INFO    winstone.Logger#logInternal: Beginning extraction from war file
  2019-12-17 05:44:40.758+0000 [id=1]     WARNING o.e.j.s.handler.ContextHandler#setContextPath: Empty contextPath
  2019-12-17 05:44:40.820+0000 [id=1]     INFO    org.eclipse.jetty.server.Server#doStart: jetty-9.4.22.v20191022; built: 2019-10-22T13:37:13.455Z; git: b1e6b55512e008f7fbdf1cbea4ff8a6446d1073b; jvm 1.8.0_222-8u222-b10-1ubuntu1~16.04.1-b10
  2019-12-17 05:44:41.192+0000 [id=1]     INFO    org.eclipse.jetty.util.TypeUtil#<clinit>: JVM Runtime does not support Modules
  2019-12-17 05:44:41.204+0000 [id=1]     INFO    o.e.j.w.StandardDescriptorProcessor#visitServlet: NO JSP Support for /, did not find org.eclipse.jetty.jsp.JettyJspServlet
  2019-12-17 05:44:41.261+0000 [id=1]     INFO    o.e.j.s.s.DefaultSessionIdManager#doStart: DefaultSessionIdManager workerName=node0
  2019-12-17 05:44:41.261+0000 [id=1]     INFO    o.e.j.s.s.DefaultSessionIdManager#doStart: No SessionScavenger set, using defaults
  2019-12-17 05:44:41.263+0000 [id=1]     INFO    o.e.j.server.session.HouseKeeper#startScavenging: node0 Scavenging every 600000ms
  2019-12-17 05:44:41.604+0000 [id=1]     INFO    hudson.WebAppMain#contextInitialized: Jenkins home directory: /var/lib/jenkins found at: EnvVars.masterEnvVars.get("JENKINS_HOME")
  2019-12-17 05:44:41.740+0000 [id=1]     INFO    o.e.j.s.handler.ContextHandler#doStart: Started w.@21422231{Jenkins v2.209,/,file:///var/cache/jenkins/war/,AVAILABLE}{/var/cache/jenkins/war}
  2019-12-17 05:44:41.770+0000 [id=1]     INFO    o.e.j.server.AbstractConnector#doStart: Started ServerConnector@3a12c404{HTTP/1.1,[http/1.1]}{0.0.0.0:8080}
  2019-12-17 05:44:41.770+0000 [id=1]     INFO    org.eclipse.jetty.server.Server#doStart: Started @2484ms
  2019-12-17 05:44:41.771+0000 [id=24]    INFO    winstone.Logger#logInternal: Winstone Servlet Engine running: controlPort=disabled
  2019-12-17 05:44:42.936+0000 [id=30]    INFO    jenkins.InitReactorRunner$1#onAttained: Started initialization
  2019-12-17 05:44:42.943+0000 [id=29]    INFO    hudson.PluginManager#loadDetachedPlugins: Upgrading Jenkins. The last running version was 2.208. This Jenkins is version 2.209.
  2019-12-17 05:44:42.975+0000 [id=29]    INFO    hudson.PluginManager#loadDetachedPlugins: Upgraded Jenkins from version 2.208 to version 2.209. Loaded detached plugins (and dependencies): []
  2019-12-17 05:44:43.752+0000 [id=30]    INFO    jenkins.InitReactorRunner$1#onAttained: Listed all plugins
  2019-12-17 05:44:53.986+0000 [id=44]    INFO    jenkins.InitReactorRunner$1#onAttained: Prepared all plugins
  2019-12-17 05:44:54.305+0000 [id=31]    INFO    jenkins.InitReactorRunner$1#onAttained: Started all plugins
  2019-12-17 05:44:54.816+0000 [id=29]    INFO    h.ExtensionFinder$GuiceFinder$FaultTolerantScope$1#error: Failed to instantiate optional component hudson.plugins.build_timeout.operations.AbortAndRestartOperation$DescriptorImpl; skipping
  2019-12-17 05:44:55.015+0000 [id=29]    WARNING jenkins.security.ClassFilterImpl#notifyRejected: java.util.concurrent.atomic.AtomicInteger in JRE might be dangerous, so rejecting; see https://jenkins.io/redirect/class-filter/
  2019-12-17 05:44:55.969+0000 [id=41]    INFO    jenkins.InitReactorRunner$1#onAttained: Augmented all extensions
  ...
  

• • Mind the line: WARNING jenkins.security.ClassFilterImpl#notifyRejected: java.util.concurrent.atomic.AtomicInteger in JRE might be dangerous, so rejecting; see https://jenkins.io/redirect/class-filter/
• In addition there are then new warnings in "Mange Old Data" section that seem to get more and more:

  hudson.plugins.git.GitSCM$DescriptorImpl	Git	ConversionException: Refusing to unmarshal generation for security reasons; see https://jenkins.io/redirect/class-filter/ ---- Debugging information ---- class : java.util.concurrent.atomic.AtomicInteger required-type : java.util.concurrent.atomic.AtomicInteger converter-type : hudson.util.XStream2$BlacklistedTypesConverter path : /hudson.plugins.git.GitSCM$DescriptorImpl/generation line number : 3
  -------------------------------
  hudson.scm.SubversionSCM$DescriptorImpl	Subversion	ConversionException: Refusing to unmarshal generation for security reasons; see https://jenkins.io/redirect/class-filter/ ---- Debugging information ---- class : java.util.concurrent.atomic.AtomicInteger required-type : java.util.concurrent.atomic.AtomicInteger converter-type : hudson.util.XStream2$BlacklistedTypesConverter path : /hudson.scm.SubversionSCM$DescriptorImpl/generation line number : 3
  

I am wondering if and what I should do, especially since the https://jenkins.io/changelog/ for v2.209 is still missing...

• Interestingly I have now received an automated "Jenkins Health Advisor by CloudBees" email pointing to:
  • https://issues.jenkins-ci.org/browse/JENKINS-47736
  • https://support.cloudbees.com/hc/en-us/articles/360001743551
• But I nonetheless guess/think that there is a general problem (outside my area) since it popped up right after upgrading Jenkins core!?