Thank you, that’s a help! It has saved me from my own blindness… 
As I said, I’m using the Linuxserver image. There I edited the site-conf/default (about two years ago) to match the container to the reverse proxy and have the correct IP in the Nextcloud logs (as described here). With one of the last updates I deleted the old file because there was a new default. Since I suspected Collabora to be the cause, I looked thoroughly in the wrong place.