====== Understanding 400 error ======
The HyperText Transfer Protocol (HTTP) 400 Bad Request response status code indicates that the server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).  The client should not repeat this request without modification.

Since the 400 Bad Request Error is a client error response code, it’s best to start by troubleshooting any potential client-side issues that could be causing this error.

For example, 
  2017-12-09 21:26:14.768 [INFO] [199.38.148.127:7822-6] Status 400: Unexpected request body 1306 bytes for request: /!

The request is a GET request but with request body, thus the web server returns a 400 status code. It is a protocol violation.  

Another example:
  2018-04-16 14:27:24.707114 NOTICE [162.158.255.25:20689] Status 400: Http request header is too big, abandon!
  2018-04-16 14:27:30.324454 NOTICE [108.162.237.139:25277] Status 400: Http request header is too big, abandon!

 As the error indicates, the http request header is too big. You can try to change **max request header size** to the maximum allowed size ''16380''.