With the following line in a plugins.txt file:
workflow-support:incrementals;org.jenkins-ci.plugins.workflow;2.19-rc289.d09828a05a74
The following URL is being used for the plugin download:
https://repo.jenkins-ci.org/incrementals/org/jenkins-ci/plugins/workflow/2.19-rc289.d09828a05a74/workflow-support-2.19-rc289.d09828a05a74.hpi
The actual URL should be:
https://repo.jenkins-ci.org/incrementals/org/jenkins-ci/plugins/workflow/*workflow-support*/2.19-rc289.d09828a05a74/workflow-support-2.19-rc289.d09828a05a74.hpi
The workflow-support is missing from after the groupId/artifactId info.
Fixed in https://github.com/jenkinsci/plugin-installation-manager-tool/pull/43