Apache Guacamole working fine over HTTP, but not over HTTPS

I've recently installed Apache Guacamole on Ubuntu 20.04 LTS and use NGINX as the proxy server. Everything works fine with it over HTTP but when I use HTTPS, the application still loads, but then the connections are super slow (get stuck for some seconds). About configuration, I've exactly done what the official documentation says and all the services run on the same server.

EDIT: NGINX Configuration:

server {
    listen 80;
    server_name  guac.example.com;
    return 301 https://$host$request_uri;
    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }
}

server {
    listen 443 ssl;
    server_name  guac.example.com;
    ssl_certificate /etc/ssl/certs/guacamole.crt;
    ssl_certificate_key /etc/ssl/private/guacamole.key;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers HIGH:!aNULL:!MD5;
    location / {
        proxy_pass http://localhost:8080/guacamole/;
        proxy_buffering off;
        proxy_http_version 1.1;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection $http_connection;
        proxy_cookie_path /guacamole/ /;
        access_log off;
    }
    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }
}

EDIT: As of the official documentation, the application uses WebSockets by default, but it will use HTTP requests if the WebSocket protocol is unavailable.

Any help is appreciated.