We upgraded Jenkins and it failed to start.
Here is the error that was displayed in the UI:

org.jvnet.hudson.reactor.ReactorException: java.lang.IllegalAccessError: tried to access method hudson.security.SecurityRealm.findBean(Ljava/lang/Class;Lorg/springframework/context/ApplicationContext;)Ljava/lang/Object; from class hudson.security.LDAPSecurityRealm$LDAPUserDetailsService
at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:246)
at jenkins.InitReactorRunner.run(InitReactorRunner.java:43)
at jenkins.model.Jenkins.executeReactor(Jenkins.java:885)
at jenkins.model.Jenkins.<init>(Jenkins.java:790)
at hudson.model.Hudson.<init>(Hudson.java:81)
at hudson.model.Hudson.<init>(Hudson.java:77)
at hudson.WebAppMain$2.run(WebAppMain.java:217)
Caused by: java.lang.IllegalAccessError: tried to access method hudson.security.SecurityRealm.findBean(Ljava/lang/Class;Lorg/springframework/context/ApplicationContext;)Ljava/lang/Object; from class hudson.security.LDAPSecurityRealm$LDAPUserDetailsService
at hudson.security.LDAPSecurityRealm$LDAPUserDetailsService.<init>(LDAPSecurityRealm.java:419)
at hudson.security.LDAPSecurityRealm.createSecurityComponents(LDAPSecurityRealm.java:369)
at hudson.security.SecurityRealm.getSecurityComponents(SecurityRealm.java:398)
at hudson.security.HudsonFilter.reset(HudsonFilter.java:134)
at jenkins.model.Jenkins.setSecurityRealm(Jenkins.java:1960)
at jenkins.model.Jenkins$17.run(Jenkins.java:2524)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:874)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)