Ok, found the issue - apparently in 2018 I manually installed a plantuml.jar into the /usr/lib/xwiki/WEB-INF/lib directory… 
I can’t remember why, maybe it was necessary back then?
In any case, it seems to have been located in the classpath before the JAR from the plugin (it actually makes sense that a plugin cannot not overwrite permanently installed JARs, from a security perspective) and thus prevented the use of a newer PlantUML version…
After I moved it away, the organisation chart and network structure diagrams work like a charm, generated locally in our wiki…
Sorry for the noise. 