You would have to check the logs on both ends to see who is logging what, run some 'ab' or curl sessions.
And use the latest version of nginx.
Curl -i should tell you when a 4xx is returned who is returning it, then check its logfiles, enabling debug logging might show some more information.
And use the latest version of nginx.
Curl -i should tell you when a 4xx is returned who is returning it, then check its logfiles, enabling debug logging might show some more information.