After upgrading Jenkins from 2.89.3 to 2.164.2 and then upgrading the Conditional Build step plugin to 1.3.6, we started seeing the following error whenever a Jenkins job is triggered using "Poll SCM".

FATAL: org/jenkinsci/plugins/conditionalbuildstep/singlestep/SingleConditionalBuilder
java.lang.NoClassDefFoundError: org/jenkinsci/plugins/conditionalbuildstep/singlestep/SingleConditionalBuilder
at com.microfocus.application.automation.tools.octane.model.processors.projects.AbstractProjectProcessor.builderClassValidator(AbstractProjectProcessor.java:189)
at com.microfocus.application.automation.tools.octane.model.processors.projects.AbstractProjectProcessor.processBuilders(AbstractProjectProcessor.java:145)
at com.microfocus.application.automation.tools.octane.model.processors.projects.AbstractProjectProcessor.processBuilders(AbstractProjectProcessor.java:131)
at com.microfocus.application.automation.tools.octane.model.processors.projects.FreeStyleProjectProcessor.<init>(FreeStyleProjectProcessor.java:40)
at com.microfocus.application.automation.tools.octane.model.processors.projects.JobProcessorFactory.getFlowProcessor(JobProcessorFactory.java:68)
at com.microfocus.application.automation.tools.octane.model.processors.projects.JobProcessorFactory.getFlowProcessor(JobProcessorFactory.java:60)
at com.microfocus.application.automation.tools.octane.tests.build.BuildHandlerUtils.getJobCiId(BuildHandlerUtils.java:138)
at com.microfocus.application.automation.tools.octane.model.CIEventFactory.createScmEvent(CIEventFactory.java:70)
at com.microfocus.application.automation.tools.octane.events.SCMListenerOctaneImpl.onChangeLogParsed(SCMListenerOctaneImpl.java:44)
at hudson.model.listeners.SCMListener.onChangeLogParsed(SCMListener.java:120)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:590)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:499)
at hudson.model.Run.execute(Run.java:1816)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:429)

I then downgraded the Conditional Build Step plugin to 1.3.5, but we are still seeing this error.  

If we run the job manually it succeeds.