During testing of JEP-200 before the release we discovered incompatibility with Monitoring plugin. Jesse Glick has fixed it on the core side by adding classes to the whitelist https://github.com/jenkinsci/jenkins/pull/3120. Then there was a fix applied in 1.71.0: https://github.com/jenkinsci/monitoring-plugin/pull/6

Unfortunately the whitelist works only for new versions of the plugin. Versions before https://github.com/jenkinsci/monitoring-plugin/commit/5cb1c662058b89830e45f30b7207796fb2090f80 in 1.68.0 fail since they use a different package for model objects.