What does this error mean with 5d?

Gani Rakhmatov asked:

I have a web server with HAproxy for load balancing and two nginx+php-fpm.
When I opened my site I saw this error. I know what does 504 error mean, but why there are 5d?

What does 5d mean ?

My answer:

Looks like something ignored or stripped out Transfer-Encoding: chunked from the response. That clearly looks like a chunked response, but delivered to the browser unchunked.

Look for something between you and the web server that might be altering your connections in flight, such as another proxy server, firewall, state censorship regime, etc.

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