Error 503 - Service temporarily unavailable

Checklist

• Have you pulled and found the error with jc21/nginx-proxy-manager:latest docker image?
  • Yes
• Are you sure you're not using someone else's docker image?
  • Yes
• Have you searched for similar issues (both open and closed)?
  • Yes

Describe the bug
I have configured NPM in the correct way when it comes to port & traffic forwarding. When I use a domain with no traffic, it works well with no issue. When I use a domain with a lot of traffic on it (700-800 clients simultaneously), I get Error 503. I am using the same domain directly without passing to the proxy, and it is working perfectly fine. When I use Proxy and I keep refreshing the website, 1/4 of times works, 3/4 of times gives error 503. I use this to hide the IP of the streaming server.

""I have checked the logs and I am getting:

183#183: *127384 connect() failed (111: Connection r
efused) while connecting to upstream, client: xxx.xxx.xxx.xxx, server: 0.0.0.0:8080,
upstream: "xx.xx.xxx.xx:8080", bytes from/to client:0/0, bytes from/to upstre
am:0/0""

Is there any way I can increase the load that it can handle? I want to set it to have no limits on traffic handling. Is there any way to do so?

Nginx Proxy Manager Version

To Reproduce
Steps to reproduce the behavior:

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4. See error

Expected behavior

Screenshots

Operating System

Additional context

Start with the reported Nginx upstream connection-refused log and the Docker deployment using the jc21/nginx-proxy-manager:latest image. Reproduce the failure under the stated 700–800 simultaneous clients, first collecting the missing Nginx Proxy Manager version, operating system, Docker details, and configuration. Done means the cause of the 503 responses and any supported traffic or configuration limit are identified.