So, as your Nextcloud instance appears to be listening for HTTP on port 443, and Caddy is opinionated against allowing HTTP upstreams on default HTTPS port, your Nextcloud installation is not supported by Caddy.
Best bet to get things running from here would be to go to Linuxserver, the maintainers of your Nextcloud image, and sort out how to fix your container so that it either accepts traffic on port 80 without issue, or listens to HTTPS instead of HTTP on port 443.