- Start Jenkins without installing any plugins
- Install an older ssh-slaves plugin (e.g. 1.14), this way there is no key verification strategy configuration. You'll need to also install ssh-credentials manually.
- Add an SSH-based agent.
- Upgrade to ssh-slaves 1.18 and restart the Jenkins instance.
- Note the following error in the agent log:
To make sure this was specific to 1.18, I tested the upgrade from 1.14 to 1.17 and did not see a NullPointerException.