I have access to a Nextcloud server and can use webdav to sync files. I tried to open tiddlywiki html file in browser via webdav. The html file is downloaded rather than opening in the browser and allowing me to edit.
As I did look at Nextcloud the last time (a year ago) there was no possibility to show a single file wiki as an html page. … IMO it’s about security restrictions.
WebDav in Nextcloud is only used to mount cloud folders as folders in the file browser, which is a completely different thing.
I use NextCloud and automatically sync the files from my private webserver to my local storage (SSD on 'puters). I open the local copy using Timimi and have no difficulties.
It’s been a while for me as well but if I remember correctly I ended up using the external sites app and the tw-reciever plugin to be able to get the function you are looking for.