Yes, you’re right. It was late when I posted the other answer and forgot to link to the guide I mentioned. This time, I used Xiao’s guide, which is simple to setup and works absolutely flawless.
In another thread, I was looking for help, somebody pointed me to the fact, that in my setup I don’t need to reverse proxy Nextcloud, only the other servers (Navidrome in my case).
I’m now using Xiao’s navidrome.conf (+ssl directives) and it works as long as I mute my Navidrome conf. If the navidrome.conf is not muted, the Nextcloud url also points to Navidrome. I guess (hope) that a small instruction in either conf file could solve this.
(I don’t post my Nextcloud config.php since I’m sure now it is a nginx and not a Nextcloud issue).
P.S.: I think I’ve solved it myself by just deleting “default server” out of the navidrome.conf and “voilà” it works. Thanks for your input.