I don't seem to be able to close this issue as "invalid" myself... (Or rather, to somehow close / cancel it at all...)
This was a user error, it turned out I had an older manually installed PlantUML jar in the classpath which jeopardized the plugins functionality.
You can cancel this issue.
https://forum.xwiki.org/t/plantuml-macro-diagram-type-issues-local-execution/16772/6?u=gohrner