@franzstein, great that you have this working! The Netatmo PWS is pretty popular and the cloud plug-in for WeewWX is definitely the simplest approach. If you have a spare moment and would be willing to post a brief set of steps that you took then I would be happy to “write it up” and add to the support page to help others.
The OS version (as opposed to OSPi) only accepts an ip/hostname in the “su” configuration popup for the weather service. It is hardcoded to port 80. You are right that the temp/humidity/precip that is shown in the System Dialog page is obtained directly from OWM rather than from the local Weather Service. We are looking to update this to show the actual values used during the water level calculation so that users can better understand the results.