Well, I did get it running. No code change it does work properly as expected when I programmed it.

I guess this is an issue with IntelliJ and not the programming.
I observed that when 'messing' around with IntelliJ and Springboot-Web there is sometimes an nginx process in the background which 'still' serves requests on the http port though the web-app with the same port is shut down (on error or properly).
I may be mistaken that the process is started by intellij i.e. the springboot app. I will keep watching this.
(Using Springboot 3.5.0, Oracle OpenJdk22, IntelliJ 2024.1.7 UE)
Thanks to all who had an eye on it.