Sorry, I haven’t noticed that there was already a very interesting thread here about that topic. I learned a lot there.
So when the /e/OS user had a ntfy installation from before and now he activates this new UnifiedPush option he will find indeed two ntfy processes running, with the same name, great.
To get a clear environment I uninstalled my ntfy installed long ago from F-Droind. As expected only one ntfy remains, the one from /e/OS.
Then, when you run a firewall like RethinkDNS and this is configured to disable network access for new processes you will find this new /e/OS ntfy still blocked. You must also enable it in the firewall.
But then it works. I received several messages with an ordinary beep. The question is still open if the coexistence of two ntfy processes was the reason for my delayed notifications.