proton_lynx , 4 months ago

Yep, definitely something wrong with the webserver 😅
Can you try this configuration?

https://nextcloud.domain.com {
        reverse_proxy 192.168.1.182:443 {
                header_up Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
                header_up X-Forwarded-Proto {scheme}
                header_up X-Forwarded-For {remote}
                header_up X-Real-IP {remote}
        }
}

You said that "originally, the Nextcloud server handled HTTPS with Let’s Encrypt at domain.com" and now you are redirecting to 192.168.1.182 on port 443. Is this Nextcloud server still serving HTTPS with Let’s Encrypt for domain.com?

I'm asking because if you are using Caddy in front of that HTTPS webserver as a reverse-proxy, you will need to override the Host header with the configured upstream address. Here's the documentation.
I think it would be something like this (?):

https://nextcloud.domain.com {
        reverse_proxy domain.com:443 {
                header_up Host {upstream_hostport}
                header_up Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
                header_up X-Forwarded-Proto {scheme}
                header_up X-Forwarded-For {remote}
                header_up X-Real-IP {remote}
        }
}