It’s officially a client installation problem now.
I tried on my laptop, also running Bullseye, but KDE Plasma instead of LXQt, and got Nextcloud added to Dolphin in barely ten seconds sing the same URL, more specifficaly, stuff.domain.tld as server, and nextcloud/remote.php/dav/user/wbk as folder, using secure connection. The resulting location in the address bar, when visiting, webdavs://stuff.domain.tld/nextcloud/remote.php/dav/files/wbk. It keeps working when adding wbk@ in front, adding :443 as port or leaving out /files/wbk at the end. It seems quite robust, so having it not work in any fashion on my laptop relieves me of a PEBKAC-error.
Maybe I’ll see whether it works on my desktop from a KDE session, else it is what it is and Ill leave it at that.
Thanks for you trouble!