Occasionally, you may run into a recurring issue with some of your websites: Nginx returns 400 Bad Request error pages to random visitors, and this only stops happening when visitors clear their cache and cookies. The error is caused by an overly large header field sent by the client. Most of the time, this is when cookie data exceeds a certain size. In order to prevent further trouble, you may simply increase the value of the large_client_header_buffers directive in order to allow a larger cookie data size:

large_client_header_buffers 4 16k;