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

【Nginx】upstream sent too big headerによる502 Bad Gatewayの解消 - エキサイト TechBlog. 2022

Syntax:	proxy_buffer_size size;
Default:	
proxy_buffer_size 4k|8k;
Context:	http, server, location

Syntax:	proxy_buffers number size;
Default:	
proxy_buffers 8 4k|8k;
Context:	http, server, location

https://nginx.org/en/docs/http/ngx_http_proxy_module.html