If Host address has a space in entry ( <space>ip or ip<space> in Host text fileld ), no connection is established and following error is visible in console.

Any spaces in front or at the end of Host address should be removed while slave setup is saved, check for invalid chars in node address should be performed.

Log from console:

[08/23/16 11:58:43] [SSH] Opening SSH connection to <space><host>

<space><host>: unknown error
ERROR: Unexpected error in launching a slave. This is probably a bug in Jenkins.
java.lang.IllegalStateException: Connection is not established!
at com.trilead.ssh2.Connection.getRemainingAuthMethods(Connection.java:1030)
at com.cloudbees.jenkins.plugins.sshcredentials.impl.TrileadSSHPublicKeyAuthenticator.getRemainingAuthMethods(TrileadSSHPublicKeyAuthenticator.java:88)
at com.cloudbees.jenkins.plugins.sshcredentials.impl.TrileadSSHPublicKeyAuthenticator.canAuthenticate(TrileadSSHPublicKeyAuthenticator.java:80)
at com.cloudbees.jenkins.plugins.sshcredentials.SSHAuthenticator.newInstance(SSHAuthenticator.java:212)
at com.cloudbees.jenkins.plugins.sshcredentials.SSHAuthenticator.newInstance(SSHAuthenticator.java:172)
at hudson.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:1212)
at hudson.plugins.sshslaves.SSHLauncher$2.call(SSHLauncher.java:711)
at hudson.plugins.sshslaves.SSHLauncher$2.call(SSHLauncher.java:706)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
[08/23/16 11:58:55] Launch failed - cleaning up connection
[08/23/16 11:58:55] [SSH] Connection closed.