What does the nginx 502 559 error code mean?
As mentioned by Richard Smith in the comment, 559 in the nginx log stands for:
the number of bytes in the HTML response that Nginx sent to the browser
Source:http://nginx.org/en/docs/http/ngx_http_log_module.html#log_format
or, as specified in the docs:
$body_bytes_sent
number of bytes sent to a client, not counting the response header; this variable is compatible with the“%B”parameter of themod_log_configApache module