Upon startup with Monitoring plugin version 1.80.0, Jenkins logs this error:
WARNING jenkins.security.ClassFilterImpl#notifyRejected: net.bull.javamelody.internal.model.HsErrPid in file:/var/cache/jenkins/plugins/monitoring/WEB-INF/lib/javamelody-core-1.80.0.jar might be dangerous, so rejecting; see https://jenkins.io/redirect/class-filter/
Apparently the latest release of the plugin has run afoul of the JEP-200 whitelist requirements.
Thanks for the issue. It's fixed by https://github.com/jenkinsci/monitoring-plugin/commit/2712582481807bce64a6137994f7c8f7c094080e and ready for the next release (1.81).