upstream sent too big header while reading response header from upstream nginx

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

http {
fastcgi_buffers 16 16k;.
fastcgi_buffer_size 32k.;
proxy_buffer_size 128.k;
proxy_buffers 4 256k;.
proxy_busy_buffers_size 256k;

Just save this and restart your nginx server. You wont face this error anymore.

victor

2X Certified Adobe Magento Commerce with 10+ years’ experience in Magento platform