I'm seeing something similar, where when I select the correct credential with the right username it works, but when I come back later the credential in the slave config has mysteriously changed to be the user of the Master and Jenkins can no longer log into the slave.

Same here. Interaction with credential plugin seems to be wrong/broken. Rollback to 0.22 solved the problem.

I've been trying the upgrade scenario today, and so far I have been unable to reproduce this problem.

I created a passphrase-protected private key and used Jenkins 1.480 (with ssh-slaves plugin 0.22) to set up a slave. The slave also uses a different user name than what the master runs.

I then upgraded to Jenkins 1.511, which uses the ssh-slaves plugin 0.23, and I see that the configuration has been successfully migrated, and the slave connected as it should be. So there must be something deeper going on that what it appears.

I do agree with the poor error message. I have just fixed this. This would let us better understand what's going wrong. This error message fix will be in 0.24.

I've released 0.24 with improved error diagnostics.

If you are seeing this problem, please upgrade to ssh-slaves 0.24 and report the error message in the slave log that you see.

Also, as we still haven't quite determined the root cause yet, please report the type of the authentication method you use (username+password? private key? if so, with or without passphrase? do you specify the user name?)

Thanks for your cooperation!

Using Jenkins 1.511 and slave-plugin 0.24.
SSH public key authentication without passphrase.

[04/17/13 08:18:57] [SSH] Opening SSH connection to limetti.x: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.

I downgraded from 0.24 to 0.23 and now it works

Joakim, are you sure 0.23 is working (as opposed to just not showing the stack trace?)
Also see JENKINS-17648

Kohsuke, I downgraded to .23 but still doesn't work.

I am using jenkins version 2.7.2. I am also facing the same issue. I am able to login using ssh through command line. But connecting to slave node from jenkins UI is not working. I tried all the options of copy paste, absolute path, from jenkins master ~/.ssh folder. I got the below error.

[08/29/16 03:59:01] [SSH] Opening SSH connection to 172.17.0.3:22.
ERROR: Server rejected the 1 private key(s) for jenkins (credentialId:f3897dd0-5808-45a5-bd3e-a7a159d6eefb/method:publickey)
ERROR: Failed to authenticate as jenkins with credential=f3897dd0-5808-45a5-bd3e-a7a159d6eefb
java.io.IOException: Publickey authentication failed.
at com.trilead.ssh2.auth.AuthenticationManager.authenticatePublicKey(AuthenticationManager.java:315)
at com.trilead.ssh2.Connection.authenticateWithPublicKey(Connection.java:467)
at com.cloudbees.jenkins.plugins.sshcredentials.impl.TrileadSSHPublicKeyAuthenticator.doAuthenticate(TrileadSSHPublicKeyAuthenticator.java:109)
at com.cloudbees.jenkins.plugins.sshcredentials.SSHAuthenticator.authenticate(SSHAuthenticator.java:415)
at com.cloudbees.jenkins.plugins.sshcredentials.SSHAuthenticator.authenticate(SSHAuthenticator.java:435)
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)
Caused by: java.io.IOException: Illegal len in DER object (1186)
at com.trilead.ssh2.crypto.SimpleDERReader.readSequenceAsByteArray(SimpleDERReader.java:136)
at com.trilead.ssh2.crypto.PEMDecoder.decode(PEMDecoder.java:355)
at com.trilead.ssh2.auth.AuthenticationManager.authenticatePublicKey(AuthenticationManager.java:224)
... 11 more
[08/29/16 03:59:01] [SSH] Authentication failed.
hudson.AbortException: Authentication failed.
at hudson.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:1217)
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)
[08/29/16 03:59:01] Launch failed - cleaning up connection
[08/29/16 03:59:01] [SSH] Connection closed.

tested on 1.26