Queue$BlockedItem.getCauseOfBlockage waiting for lock on Queue

"Handling POST /job/.../ajaxBuildQueue : ...
   java.lang.Thread.State: BLOCKED (on object monitor)
	at hudson.model.ResourceController.getBlockingActivity(ResourceController.java:129)
	- waiting to lock <0x00000000c11cf910> (a hudson.model.Queue)
	at hudson.model.Queue$BlockedItem.getCauseOfBlockage(Queue.java:1490)
	at hudson.model.Queue$Item.getWhy(Queue.java:1309)
	at sun.reflect.GeneratedMethodAccessor681.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)