For those looking for a solution, figured it out after quite some research and trial&error:
You have to register a http-callback on the bridge using the hostname of the Node-RED machine including the port.
for example
http://xxx.yyy.zzz:1880/nuki
Only with the port included you can intercept the opener ring message in Node-RED via a http-in node.
My error was that I had omitted the port when registering the callback url in the bridge.