So I have been able to figure out what causes the problem: I was connecting to NextCloud with my browser by using localhost instead of the IP address. The browser used this url for sending the request to CODE, which then tried to reply to localhost. Accessing to NextCloud with the ip address of my machine fixes everything.
I understand it’s my fault, I overlooked point 3 of the quick tryout guide and assumed that Note 2 was intended just for the Collabora server settings and not for the browser url. I’m really sorry for the noise, thank you for your patience.