I think I’ve found the issue, it looks identical to the issue reported here.
Nextcloud pass through settings blank
This may be one for them, but I thought I’d start here. I logged my Android Nextcloud app into the server, it works fine. I then click into settings and Set up Davx, it goes over to the Davx app, but I just get a blank window where I would expect a log in to appear.
I did the same thing with another Nextcloud isntance, and that one worked. The only difference I can think is the one that doesn’t work has a self signed certificate for HTTPS.
What do your Nextcloud / Web server logs say?
If there’s nothing useful, please ask in the Nextcloud forums, because this sounds like a server thing.