When using the Pipeline API download with the flat option, the plugin throws the exception mentioned in the Summary. It doesn't matter what value you set flat to; its presence in the downloadSpec map triggers the error. E.g.,
The same goes for the other boolean option, recursive.
I believe this is because, at least in the jenkinsci repo, GenericDownloadExecutor is importing jgit.util.StringUtils while the dependency isn't actually packaged up with the current plugin.
I have a sneaking suspicion
HAP-823 may refactor away this bug, but am unsure.
FWIW another user reported this as well on SO.