When the plugin throws an error when trying to execute the ssh-agent in a pipeline, it can be the case that there is no reason in the error log. 

hudson.AbortException: Failed to run ssh-agent: 
	at PluginClassLoader for ssh-agent//com.cloudbees.jenkins.plugins.sshagent.exec.ExecRemoteAgent.<init>(ExecRemoteAgent.java:55) 

Without any reason, it is nearly impossible to debug this. Which brings me to a second point: How am I supposed to debug this now, with no useful error output?