after upgrading from 1.500 to 1.521

slave unstable become online and offline with the rule "Take this slave on-line when in demand and off-line when idle".

Sorry, i cant found any log, except

JNLP agent connected from /<ip_address>
<===[JENKINS REMOTING CAPACITY]===>
This is a Unix slave
Slave successfully connected and online
ERROR: Connection terminated
ha:AAAAWB+LCAAAAAAAAP9b85aBtbiIQSmjNKU4P08vOT+vOD8nVc8DzHWtSE4tKMnMz/PLL0ldFVf2c+b/lb5MDAwVRQxSaBqcITRIIQMEMIIUFgAAckCEiWAAAAA=java.net.SocketException: Socket closed
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:129)
at java.io.BufferedInputStream.fill(BufferedInputStream.java:218)
at java.io.BufferedInputStream.read(BufferedInputStream.java:237)
at java.io.ObjectInputStream$PeekInputStream.peek(ObjectInputStream.java:2249)
at java.io.ObjectInputStream$BlockDataInputStream.peek(ObjectInputStream.java:2542)
at java.io.ObjectInputStream$BlockDataInputStream.peekByte(ObjectInputStream.java:2552)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1297)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
at hudson.remoting.Command.readFrom(Command.java:92)
at hudson.remoting.ClassicCommandTransport.read(ClassicCommandTransport.java:72)
at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:48)

slave run on AIX, 6.1
with the java 1.5 default, but 1.6 also installed.

<slave>
<name>4appt</name>
<description></description>
<remoteFS>/home/co/jenkins</remoteFS>
<numExecutors>2</numExecutors>
<mode>NORMAL</mode>
<retentionStrategy class="hudson.slaves.RetentionStrategy$Demand">
<inDemandDelay>1</inDemandDelay>
<idleDelay>1</idleDelay>
</retentionStrategy>
<launcher class="hudson.slaves.CommandLauncher">
<agentCommand>ssh co@<ip_address> "cd /home/co/jenkins && /usr/java6_64/jre/bin/java -Dfile.encoding=UTF-8 -jar slave.jar -jnlpUrl http://cpapp:10599/computer/4appt/slave-agent.jnlp -secret <hexamail_string>"</agentCommand>
</launcher>
<label></label>
<nodeProperties>
<hudson.slaves.EnvironmentVariablesNodeProperty>
<envVars serialization="custom">
<unserializable-parents/>
<tree-map>
<default>
<comparator class="hudson.util.CaseInsensitiveComparator"/>
</default>
<int>2</int>
<string>LANG</string>
<string>RU_RU.UTF-8</string>
<string>PATH</string>
<string>/usr/java6_64/jre/bin/:/usr/java6_64/bin/:$PATH</string>
</tree-map>
</envVars>
</hudson.slaves.EnvironmentVariablesNodeProperty>
</nodeProperties>
</slave>

from http://SERVER/systemInfo:

java.class.path jenkins.zip
java.class.version 50.0
java.endorsed.dirs /usr/java/jdk1.6.0_18/jre/lib/endorsed

Please, request, if needed any additional information - I'll add it.