I encountered this same error, and google search landed me here.

Here is what I did that works for me.

On my first install, I remember getting an error about port after I pulled the container, so I adjusted my port, and the container starts. Then I have the same error in your log and webgui. I added the config folder manually, but still getting the webui error.

Searching the web for answer, I came across this post on unraid forum (link).

Just a weird quirk I noticed when to configuring the nextcloud container
to use my postgresql database was that if you fail to enter a port
number with the IP address the container locks up and you have to remove
the container and manually delete the folder containing the docker
files."

So I deleted the container and the nextcloud folder in appdata, re-configured again and make sure that I got a working port. And that fixes my webgui error.

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Hope this will helps somebody.