Here's how to reproduce:

1. Create Plan A that has two stages. One job in Stage 1 creates a shared artifact, another job in Stage 2 downloads the artifact using the Artifact download task.

2. Clone Plan A into Plan B.

3. To make sure Stage 2 of Plan B uses artifacts from Plan B and not from Plan A, edit the artifacts downloaded by the Artifact download task. Change source plan from Plan A to Plan B. The contents of the Artifact name combo box stays the same (which kind of makes sense, because the plan is a clone), so just hit Save.

4. Run Plan B. Stage 2 of Plan B does not start to execute displaying a message: "Job has not yet been queued. Waiting for prior stages to complete", while the previous stage has already completed. The build hangs. Stop the build to see an error saying "Unable to find an artifact definition for name: <your artifact name here>", while the artifact is clearly there, possible to download from Stage 1 job's Artifacts tab.

5. Further builds of the plan also result in the same error.

The current workaround to fix Plan B definition is to first change the plan from which the artifact is to be downloaded, hit Save. Then again, edit the artifact download task parameteres and choose the actual artifact to download, hit Save. It looks like the artifact name combo box is not properly refreshed when changing the source plan.

Another possible issue with the Artifact name combo box not refreshing properly is that when one changes the source plan and hit Save, the downloader will get all the artifacts and not the one that was indicated in the combo box when hitting Save.