I've found the same problem with current stable Nginx-1.14.1 from nginx-repo on CentOS6. There are two web-sites on this server, and problem occurs only on one of them. Also I have many servers with nginx without this problem, so the problem may have some site-specific reasons, but I can't find any differences or strangeness in this environment. The problem was resolved by installing Ngby tekuchev - Other discussion