If you're using Symfony framework: Before messing with Nginx config, try to disable ChromePHP first.
1 - Open app/config/config_dev.yml
2 - Comment these lines:
#chromephp:
#type: chromephp
#level: info
ChromePHP pack the debug info json-encoded in the X-ChromePhp-Data header, which is too big for the default config of nginx with fastcgi.
Source: https://github.com/symfony/symfony/issues/8413#issuecomment-20412848