Web-DAV asks for Credentials but has never been setup

[j-ed]

That’s strange. I would assume that you’ve run into a QNAP specific configuration issue of the web server. Unfortunately I didn’t know QNAP devices and couldn’t help you on this. Have you seen this thread which covers the installation on a QNAP NAS:

Installing on QNap NAS?

The thing about Docker containers are that they have no updating mechanism like snaps have. For ease of future maintenance (trying to think longer term), I wanted to install a snap, not a Docker container, on a QNAP TS-251A, and I was able to do it. I installed the Nextcloud 13 “Candidate” snap (talked about here). To use snapd, I installed it inside a Linux Virtual Machine (using Debian 9.3, which was current at the time of installation), which I created from scratch on the QNAP . Perhaps …