You might have experienced this when you have a Nginx server for Magento >=2.3.5 onwards.
You have a very easy solution for this just add this to your nginx conf
usually you can find this configuration under this path /etc/nginx/nginx.conf
fastcgi_buffers 16 16k;.
proxy_buffers 4 256k;.
Just save this and restart your nginx server. You wont face this error anymore.
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