Suddenly we see a lot of those exceptions. After it happens no build can be scheduled anymore.

Aug 24, 2015 5:00:04 PM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.model.Queue$MaintainTask@1f721901 failed
java.lang.ClassCastException: hudson.plugins.batch_task.BatchTask cannot be cast to hudson.model.AbstractProject
at hudson.plugins.buildblocker.BuildBlockerQueueTaskDispatcher.getBuildBlockerProperty(BuildBlockerQueueTaskDispatcher.java:199)
at hudson.plugins.buildblocker.BuildBlockerQueueTaskDispatcher.canTake(BuildBlockerQueueTaskDispatcher.java:108)
at hudson.model.Queue$JobOffer.canTake(Queue.java:300)
at hudson.model.Queue.maintain(Queue.java:1476)
at hudson.model.Queue$MaintainTask.doRun(Queue.java:2576)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:51)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:304)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:178)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:744)