Since jenkins 2.34 (issue https://issues.jenkins-ci.org/browse/JENKINS-38867), jobs that use a label expression to restrict where to run, do not start anymore. In the jenkins log, the follow error is seen:

Jun 25, 2017 9:27:07 AM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.model.Queue$MaintainTask@61b3a527 failed
java.lang.UnsupportedOperationException
at java.util.Collections$UnmodifiableCollection.removeAll(Collections.java:1068)
at com.sonyericsson.jenkins.plugins.externalresource.dispatcher.ExternalResourceQueueTaskDispatcher.getReservedExternalResourceAction(ExternalResourceQueueTaskDispatcher.java:169)
at com.sonyericsson.jenkins.plugins.externalresource.dispatcher.ExternalResourceQueueTaskDispatcher.canTake(ExternalResourceQueueTaskDispatcher.java:69)
at hudson.model.Queue$JobOffer.canTake(Queue.java:259)
at hudson.model.Queue.maintain(Queue.java:1526)
at hudson.model.Queue$MaintainTask.doRun(Queue.java:2716)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:50)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)