Cordova-plugin-websocket-server not working with Ionic5


we’re started developing with Ionic recently and I’m trying to get the cordova-plugin-websocket-server working. Seems the plugin is starting correctly, but the subscriptions for watchMessages or watchOpen are never called. Anyway I can see in the console, that the plugin itself logs connections and received messages (I used, but the callbacks are never called. I also tried to hook to the handlers of the plugin directly by bypassing @ionic-native//web-socket-server and calling cordova.plugins.wsserver.start directly with no success.
Should the plugin work with ionic5?
Setup: Ionic5, capacitor (all current versions), iOS and Android apps, both simulator and deployed apps.

Thanks for help or hints.

Btw, I also tried cordova-plugin-webserver, same problem.