The problem is that /-o will also match something like --my-irrelevant-option.

Word boundaries match the end (or the beginning) of the word.

How exactly to do it depends on the regex library, my less is built with PCRE2 therefore I can do /-o\b.

Seems like the router doesn’t like how the headers are passed on. You could try:

login.router.lan {
    reverse_proxy 192.168.1.1:80 {
        header_up Host {upstream_hostport}
    }
}

https://caddyserver.com/docs/caddyfile/directives/reverse_proxy#headers

IIRC some plugins stopped returning results through the VPN interface that qbittorrent was bound to. I moved on to other methods.