This error indicates that the Jenkins master running at the given URL has never configured, and therefore the swarm client cannot determine its HTTP URL. To fix this problem, please go to the system configuration screen of this Jenkins, and click "submit".
Version 1.20 (2014 Oct 8)
Version 1.19 (2014 Oct 6)
- Correct bug introduced by 1.18 where the client did not work _unless_ you set tool locations (#24995, see pull request 18)