When I add a REST service to this demo app and follow the advice given in Confluence 8.9 release EAP available now - #27 by SujayCHegde then I can call the the service from my other app without issues.
The Listener still complains about classloading issues when he tries to process the event. But if I remove the listener, no issues are logged by Confluence.
I still cannot explain any of this and it may be that not everything in the configuration is necessary, but I hope that someone also struggling with classloading issues on Confluence 9 finds this helpful … (even if the root cause is probably a comma in some XML … 
)