I upgraded Jenkins to version 1.529, Credentials Plugin to 1.7.6 and Jenkins SSH Slaves plugin to 1.2 and I got the following error when Jenkins tried to open SSH connection some hosts but not others:

[09/02/13 02:28:00] [SSH] Opening SSH connection to hostess:22.
hudson.AbortException: Cannot find SSH User credentials with id: 33a7c255-7edb-3214-6679-67798ae456b4
at hudson.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:989)
at hudson.plugins.sshslaves.SSHLauncher.launch(SSHLauncher.java:531)
at hudson.slaves.SlaveComputer$1.call(SlaveComputer.java:230)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)
[09/02/13 02:28:00] [SSH] Connection closed.