All the slots on the slaves show up as dead. Drilling down this is the exception:

java.lang.NoSuchMethodError: hudson.model.Queue$Executable.getParent()Lhudson/model/Queue$Task;
at hudson.plugins.throttleconcurrents.ThrottleQueueTaskDispatcher.buildsOfProjectOnNode(ThrottleQueueTaskDispatcher.java:104)
at hudson.plugins.throttleconcurrents.ThrottleQueueTaskDispatcher.canTake(ThrottleQueueTaskDispatcher.java:39)
at hudson.model.Queue$JobOffer.canTake(Queue.java:202)
at hudson.model.Queue.pop(Queue.java:750)
at hudson.model.Executor.grabJob(Executor.java:175)
at hudson.model.Executor.run(Executor.java:113)