The usage of the environment variable JAVA_HOME (set via Node Properties) to specify the Java installation path results in the following exception when connecting:

ERROR: Unexpected error in launching a agent. This is probably a bug in Jenkins.
java.lang.UnsupportedOperationException
{{ at java.util.AbstractList.add(Unknown Source)}}
{{ at java.util.AbstractList.add(Unknown Source)}}
{{ at hudson.plugins.sshslaves.DefaultJavaProvider.lookForJavaHome(DefaultJavaProvider.java:94)}}
{{ at hudson.plugins.sshslaves.DefaultJavaProvider.getJavas(DefaultJavaProvider.java:76)}}
{{ at hudson.plugins.sshslaves.JavaVersionChecker.resolveJava(JavaVersionChecker.java:72)}}
{{ at hudson.plugins.sshslaves.SSHLauncher$2.call(SSHLauncher.java:851)}}
{{ at hudson.plugins.sshslaves.SSHLauncher$2.call(SSHLauncher.java:821)}}
{{ at java.util.concurrent.FutureTask.run(Unknown Source)}}
{{ at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)}}
{{ at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)}}
{{ at java.lang.Thread.run(Unknown Source)}}

I guess this is related to the following code, which tries to add a path to an immutable list:

    private List<String> lookForJavaHome(Node node) {
        List<String> ret = Collections.emptyList();
        if(node != null && node.getNodeProperties() != null){
            for (NodeProperty property : node.getNodeProperties()){
                if(property instanceof EnvironmentVariablesNodeProperty){
                    EnvVars env = ((EnvironmentVariablesNodeProperty) property).getEnvVars();
                    if (env != null && env.containsKey(JAVA_HOME)) {
                        ret.add(env.get(JAVA_HOME) + BIN_JAVA);
                    }
                }
            }
        }
        return ret;
}

https://github.com/jenkinsci/ssh-slaves-plugin/blob/master/src/main/java/hudson/plugins/sshslaves/DefaultJavaProvider.java