How to set up a new backup on Nextcloud

Hello,
I installed Nextcloud on a home server machine running with Yunohost. Yunohost offers a domain name “xxxxxx.ynh.fr” and Nextcloud gives me “https://xxxxxx.ynh.fr/nextcloud/remote.php/webdav/” as the address to access my files through WebDAV.
I can’t create a new backup despite my various attempts. I get the following error “Failed to connect: Error: TrustFailure (Authentication failed, see inner exception.)” when I test the connection with the following parameters :
Storage type: WebDAV
Use SSL: OK
Server and port: https://xxxxxx.ynh.fr
Path on the server: nextcloud/remote.php/webdav/Test
Username: my user for Yunohost
Password: the user’s password

I have reached my level of incompetence… Any help is welcome

Sincerely

Storage type: WebDAV
Use SSL: OK
Server and port: xxxxxx.ynh.fr/nextcloud/remote.php/webdav
Path on the server: Test
Username: my user for Yunohost
Password: the user’s password

Thank you for your answer.
I still have the same error with the parameters you tell me when I test the connection.
When I uncheck “Use SSL”, I get the error "Failed to connect: The remote server returned an error: (503) Service Unavailable. "

Any idea?

Edit : This works by unchecking “Use SSL” and removing the indication port 443.

Looks like a certificate problem. Connection works fine, but your Duplicati end doesn’t trust Nextcloud hosting SSL certificates, thus this TrustFailure error. I assume you’re accessing your Nextcloud instance with your browser and perhaps some desktop client, so they recognize nextcloud’s certificate properly. If that’s the case, it looks like a mono problem. In my system (Ubuntu) there’s a package called ca-certificates-mono. Perhaps it’s missing?

Here’s more info about mono, certificates, and how to troubleshoot them.

Hope it helps!

Thank you for your message.
The ca-certificates-mono package is well installed on my client machine and I just installed it on the server machine. Unfortunately, I still have the same mistake…
I will study with my little knowledge your link.
Thank you again for your help