Reproduction steps:
// I ran it with mvn hpi:run with the newest version of the docker plugin

• from Jenkins dashboard, click 'Build Executor Status'
• click ‘New Node’ button
• name it anything, click ‘Permanent Agent’ option
• I put the remote root directory as /tmp and left everything else as is
• click ‘Save' and boom, angry Jenkins

Theory:

• it is due to the launch method ‘Docker variant of Launch agents via SSH awith SSH key injection’ - does not fail gracefully when given incorrect details
• I played with leaving the host empty and typing in something, and either way got the same stack trace

Stack trace is attached.