My guess you are using Ngnix. Is this correct? What is your PHP version? Can you PM access to your server? I will investigate.
Yes, NGINX.
nginx/1.12.1
PHP 7.0.25 (cli) (built: Oct 24 2017 17:24:01) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2017 Zend Technologies
with Zend OPcache v7.0.25, Copyright (c) 1999-2017, by Zend Technologies
My server doesn't have FTP though. Is there something else that I can send you?
NGINX Rules
pastebin . com/NF6F8cc3
Tried to disabled http2 and pagespeed but still the same.