After the upgrade today to:
Jenkins: 1.526
Credentials: 1.6
SSH Slaves: 0.27
SSH Credentials: 1.0

There is no connection possible to SSH slaves. The credentials drop down box first was empty. I figured out the only credentials shown are SSH Key credentials. username and password credentials where not shown and not found, since they were added by the migration process.

After creating SSH key credentials the ssh slaves fail with:

[08/08/13 15:26:30] [SSH] Opening SSH connection to abc.intra:22.
ERROR: Unexpected error in launching a slave. This is probably a bug in Jenkins.
java.lang.NoSuchMethodError: com.cloudbees.jenkins.plugins.sshcredentials.SSHAuthenticator.newInstance(Ljava/lang/Object;Lcom/cloudbees/jenkins/plugins/sshcredentials/SSHUser;)Lcom/cloudbees/jenkins/plugins/sshcredentials/SSHAuthenticator;
at hudson.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:949)
at hudson.plugins.sshslaves.SSHLauncher.launch(SSHLauncher.java:488)
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)
[08/08/13 15:26:30] [SSH] Connection closed.