The error log from <master>/log/all shows this exception:

Feb 19, 2015 7:31:00 PM WARNING hudson.remoting.AbstractByteArrayCommandTransport
Failed to construct Command
java.io.EOFException
	at java.io.ObjectInputStream$PeekInputStream.readFully(ObjectInputStream.java:2325)
	at java.io.ObjectInputStream$BlockDataInputStream.readShort(ObjectInputStream.java:2794)
	at java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:801)
	at java.io.ObjectInputStream.<init>(ObjectInputStream.java:299)
	at hudson.remoting.ObjectInputStreamEx.<init>(ObjectInputStreamEx.java:40)
	at hudson.remoting.AbstractByteArrayCommandTransport$1.handle(AbstractByteArrayCommandTransport.java:61)
	at org.jenkinsci.remoting.nio.NioChannelHub$2.run(NioChannelHub.java:584)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
	at java.util.concurrent.FutureTask.run(FutureTask.java:262)
	at hudson.remoting.SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:111)
	at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
	at java.util.concurrent.FutureTask.run(FutureTask.java:262)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)