I am getting the following in the Nginx logs (because the WordPress site I am debugging for a client is generating tons of cookies):

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

The solution is here, e.g. To add the proxy_buffer_size configurations to the location block:

server { listen 80; server_name host.tld; location / { proxy_pass http://upstream; ... proxy_buffer_size 128k; proxy_buffers 4 256k; proxy_busy_buffers_size 256k; } }