After upgrading the SSH Slaves Plugin from version 0.23 to 0.24 I the following exception while trying to connect slaves:

[04/17/13 08:18:57] [SSH] Opening SSH connection to XXX:22.
ERROR: Unexpected error in launching a slave. This is probably a bug in Jenkins.
hudson.security.AccessDeniedException2: anonymous is missing the Administer permission
at hudson.security.ACL.checkPermission(ACL.java:54)
at hudson.model.Node.checkPermission(Node.java:394)
at com.cloudbees.plugins.credentials.SystemCredentialsProvider.save(SystemCredentialsProvider.java:178)
at hudson.plugins.sshslaves.SSHLauncher.upgrade(SSHLauncher.java:407)
at hudson.plugins.sshslaves.SSHLauncher.getCredentials(SSHLauncher.java:351)
at hudson.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:907)
at hudson.plugins.sshslaves.SSHLauncher.launch(SSHLauncher.java:473)
at hudson.slaves.SlaveComputer$1.call(SlaveComputer.java:223)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)
[04/17/13 08:18:57] [SSH] Connection closed.