OK, managed to add a device. My confusion came from the fact that my Syncthing installation is very slow in reacting. My web-page (:8080) becomes inactive and reloads without my doing anything. Is there anything I can do about that?
Every time I add a device or a folder, I get an error that there was a connection problem. I am on a local LAN with my desktop/browser connecting to the syncthing server on the NAS. For the rest, my NAS works well. So, overall syncthing is unstable for now.