After shifting webserver to Nginx, if you received error "upstream sent too big header while reading response header from upstream" then Please below steps.
- Login to the Plesk panel.
- Hosting & DNS --> Apache & nginx Settings
- Find 'Additional nginx directives' and add below values.
- fastcgi_buffers 16 16k;
fastcgi_buffer_size 32k;
proxy_buffer_size 128k;
proxy_buffers 4 256k;
proxy_busy_buffers_size 256k;