I suspect the Delivery Pipeline Plugin is not really causing this issue.
I started the plugin in debug again in Eclipse. Re-created the job using the same script from my previous commits. Installed the Delivery Pipeline plug-in. Restarted Jenkins. Configured the job to include the Delivery Plugin configuration, with some dummy values.
Everything works fine.
I'll have to work on other issues in this development cycle, and likely won't have time to debug this issue again, unless there is a definite way to reproduce it in my environment (or if there is a pull request ). Will start another development cycle near December anyway, so we can fix it before x-mas and cut a release.