Are you handling the Upgrade Header in the proxy? I know in NGINX (acting as a reverse proxy) I had to add

[chenkianwee]

          Are you handling the Upgrade Header in the proxy?  I know in NGINX (acting as a reverse proxy) I had to add

    proxy_set_header            Upgrade $http_upgrade;
    proxy_set_header            Connection "upgrade";

My full nginx config looks like:

server {
  listen                        80;
  server_name                   bim.domain.com;
  return                        301 https://$host$request_uri;
}

server {
  listen                        443 ssl;
  server_name                   bim.domain.com;
  ssl_certificate               /etc/nginx/certs/domain.crt;
  ssl_certificate_key           /etc/nginx/certs/domain.key;

  location / {
# TODO    return                      301 https://bim.domain.com/bimserver;
  }

  location /bimserver {
    proxy_redirect          off;
    proxy_set_header        Host $host;
    proxy_set_header        X-Real-IP $remote_addr;
    proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header        X-Forwarded-Proto $scheme;
/etc/nginx/proxy.conf (END)

    proxy_pass                  http://0.0.0.0:8080/bimserver;
    proxy_set_header            Upgrade $http_upgrade;
    proxy_set_header            Connection "upgrade";
  }

_Originally posted by @coryasilva in https://github.com/opensourceBIM/BIMserver/issues/569#issuecomment-308630912_