Sinatra + Thin + Nginx connect() failed (111: Connection refused) while connecting to upstream

I have a Sinatra app that is running on Thin with Nginx as a reverse proxy and receives a lot of traffic. My users are reporting 502 errors and looking at the Nginx logs i see a lot of these:

[warn] upstream server temporarily disabled while connecting to upstream
[error] connect() failed (111: Connection refused) while connecting to upstream

If i look at the logs from the Sinatra app i see no errors.

I am starting Thing with the following:

--max-conns 15360 --max-persistent-conns 2048 --threaded start

I have set the following for Ninx:

worker_processes  auto;
worker_rlimit_nofile 65535;

events {
    worker_connections  15360;
}

The host file for the Sinatra app:

server {
    server_name my_sinatra_app;

    #lots of bots try to find vulnerabilities in php sites
    location ~ \.php {
        return 404;
    }

    location / {
        proxy_pass http://localhost:6903;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_cache_bypass $http_upgrade;

        #increase buffers
        proxy_buffer_size          128k;
        proxy_buffers              4 256k;
        proxy_busy_buffers_size    256k;
    }

    listen 443 ssl; # managed by Certbot
    #...
    #SSL stuff
}

Why is this happening? Too much traffic?

What's the solution? Do I keep increasing the worker_connections and --max-conns until the errors stop?

The output of htop seems like the server can handle more:

Any insight/advice?

EDIT

While i don't see any errors in the Sinatra log or systemctl status output, i did notice that the service never runs for very long so it seems Thin server is crashing often. Any idea how i can debug this further?

So the problem was actually with Thin server, for some reason it kept crashing every few minutes with a C++ error and therefore Nginx would throw those errors while attempting to connect to Thin and failing (because Thin would be crashing/rebooting).

The solution was to replace Thin with Puma, after that no more issues.