Unfortunately this is not a Traefik issue but a Silverbullet issue. Essentially, the web server doesn't support a subpath and instead assumes that it is at the root - see https://github.com/silverbulletmd/silverbullet/issues/794 for details.
You can try hosting it from a different subdomain or port instead.
You need a domain name that will resolve to the IP address of the machine running the server. It doesn't need to be your own DNS server. If you already have a domain name, you can just add an A record that points to the IP in your local network.