I have set up the new QGIS Web Client 2 (QWC2) (https://github.com/qgis/qwc2-demo-app) including the demo app and got it running quite easily.
I have created a QGIS Server WMS and also can include it in other QGIS Projects as WMS. So the WMS itself is working. The project's CRS is set to EPSG::25833.
In the qwc2-demo-app-master -> themesConfig.json I have replaced the first theme with my own WMS and specified the EPSG according to my project (EPSG::25833).
I "yarn start" the WebClient and it shows "webpack: Comlied successfully". The WebClient shows up on localhost:8081 and also my own project shows up, since it is set to default. It is also displayed in themes and legends, the features however are NOT displayed on the map.
I'm stuck at this point and tried to change the QGIS Server CRS to EPSG::4326... but my features never show up on the map.
I cannot find much about this relatively new WebClient development.
I reproduced the exact same procedure on another machine and it works. However, as soon as I add another Layer to my QGIS Project (the serving WMS layer) is is not showing any feature any more at all... clueless!