I just read the September updates blog post and updated via App Center. I don't see anything added to Online Accounts; so, I'm assuming I still connect to the Nextcloud server through Files>Connect to server. When I do that, I still get the same behavior as I previously would which was, after entering the information, the configuration screen comes back and nothing connects.
While nothing to offer in regards to the libcloudproviders update to files (because it makes no difference here, on my system, at all)
It sounds like you would like to connect to nextcloud via Webdav. Let me help you do that.
Open files, and click -> Connect to Server
choose whichever fits (Https if you have set up nextcloud with SSL or HTTP if not)
in the serverfield input: url or ip address as appropriate. no protocol spec. e.g.:
then under dir/folder, input:
then input your username/password. If you set it up with a selfsigned certificate, you will be prompted to accept an untrusted certificate. Then it should be connected. but please notice: This webdav connection will not be seen by files as anything related to nextcloud, as that is abstracted away when using something like webdav (and that is really what such protocols are for)