Registration Failure: Registration is not supported by the server

[ljrobins]

I'm very excited to get Snikket working on my home server. Currently, I'm running behind nginx with the http block:

  # Accept HTTP connections
  listen 80;
  listen [::]:80;

  server_name chat.mydomain.com;
  server_name groups.chat.mydomain.com;
  server_name share.chat.mydomain.com;

  location / {
      proxy_pass http://192.168.4.xx:5080/;
      proxy_set_header      Host              $host;
      proxy_set_header      X-Forwarded-For   $proxy_add_x_forwarded_for;

      # This is the maximum size of uploaded files in Snikket
      client_max_body_size 104857616; # 100MB + 16 bytes
  }
}```

And the https block:

```# snikket
server {
  # Accept HTTPS connections
  listen [::]:1443 ssl ipv6only=on;
  listen 1443 ssl;
  ssl_certificate /etc/letsencrypt/live/mydomain.com/fullchain.pem;   # managed by certbot on host machine
  ssl_certificate_key /etc/letsencrypt/live/mydomain.com/privkey.pem; # managed by certbot on host machine

  server_name chat.mydomain.com;
  server_name groups.chat.mydomain.com;
  server_name share.chat.mydomain.com;

  location / {
      proxy_pass https://192.168.4.xx:5443/;
      proxy_set_header      Host              $host;
      proxy_set_header      X-Forwarded-For   $proxy_add_x_forwarded_for;
      # REMOVE THIS IF YOU CHANGE `localhost` TO ANYTHING ELSE ABOVE
      # proxy_ssl_verify      off;
      proxy_set_header      X-Forwarded-Proto https;
      proxy_ssl_server_name on;

      # This is the maximum size of uploaded files in Snikket
      client_max_body_size 104857616; # 100MB + 16 bytes

      # For BOSH and WebSockets
      proxy_set_header Connection $http_connection;
      proxy_set_header Upgrade $http_upgrade;
      proxy_read_timeout 900s;

  }
}

I am using sslh to route the XMMP traffic (docs do not show how to do the same for STUN/TURN, that's next on the list to get figured out). Here's the sslh config, it comes directly from the docs:

(
    { host: "0.0.0.0"; port: "443"; },
);

protocols:
(
     ## Snikket rules
     # Send encrypted XMPP traffic directly to Snikket (this must be above the HTTPS rules)
     { name: "tls";     host: "127.0.0.1"; port: "5223"; alpn_protocols: [ "xmpp-client" ]; },
     # Send HTTPS traffic to Snikket's HTTPS port
     { name: "tls";     host: "127.0.0.1"; port: "5443"; sni_hostnames:  [ "chat.mydomain.com", "groups.chat.mydomain.com", "share.chat.mydomain.com" ] },
     # Send unencrypted XMPP traffic to Snikket (will use STARTTLS)
     { name: "xmpp";    host: "127.0.0.1"; port: "5222"; },

     ## Other rules
     # Add rules here to forward any other hosts/protocols to non-Snikket destinations
     { name: "tls";     host: "127.0.0.1"; port: "1443"; },
);

Running sslh with:

  sslh:
    image: ghcr.io/yrutschle/sslh:latest
    container_name: sslh
    network_mode: "host"
    volumes:
      - ./sslh.conf:/etc/sslh.conf:ro
    cap_add:
      - NET_ADMIN
    command: >
      --foreground
      -F /etc/sslh.conf
    restart: unless-stopped

The problem:

The web interface loads just fine, but whenever I generate and use an admin invite link (on the iOS app) I get:
"Registration Failure: Registration is not supported by the server". I can't find mention to this error message anywhere on this repo or the iOS app repo, or on the docs. Thankful for any guidance anyone can provide!