This error during backup is almost certainly caused by a wrong SSL certificate: either correct but expired one, or a self-signed one, or belonging to a different domain. You can always check it by downgrading your Nextcloud instance to HTTP and re-adding the Nextcloud account again. If it works with http://… then you know where to look. As a bonus it’s also very easy to capture unencrypted http traffic with tcpdump to see what DAV commands exactly are being issued and what the server responses are.
Do have in mind that this setup should only be used for testing purposes, as you’re going to be sending your password unencrypted over the net. After the testing is done, change your Nextcloud password that is used for Sailfish backups.