Slave is connected using JNLP. I have changed the availability from on demand to "activate with a schedule" and after saving I got a page with the following message:

Connection was broken

java.net.SocketException: socket closed
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(Unknown Source)
at java.io.BufferedInputStream.fill(Unknown Source)
at java.io.BufferedInputStream.read(Unknown Source)
at java.io.ObjectInputStream$PeekInputStream.peek(Unknown Source)
at java.io.ObjectInputStream$BlockDataInputStream.peek(Unknown Source)
at java.io.ObjectInputStream$BlockDataInputStream.peekByte(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at hudson.remoting.Channel$ReaderThread.run(Channel.java:852)

Then after a while I updated the status page and the slave was on-line again.