Posted it in the wrong spot earlier
I’m presuming you have the same issue as iOS: CalDav/CardDAV: domain daaccountvalidationdomain / error 100 and How to resove well-know caldav and carddav Security & Set Up Warning
I just fixed this today and the way I determined the issue was to do
curl -v https://[redacted]/.well-known/carddav
In my case before the fix I noticed that the redirect was
Location: http://[redacted]/remote.php/dav
Note it says http rather than HTTPS.
So I modify my Caddyfile to have explicit redirects to the HTTPS
redir /.well-known/carddav https://[redacted]/remote.php/dav 301
redir /.well-known/caldav https://[redacted]/remote.php/dav 301
redir /.well-known/webfinger https://[redacted]/index.php/.well-known/webfinger 301
redir /.well-known/nodeinfo https://[redacted]/index.php/.well-known/nodeinfo 301