I figured it out just now. A little nginx config change.
I saw this in the nginx log:
2019/02/26 00:36:25 [error] 28692#28692: *61553 upstream sent too big header while reading response header from upstream, client: xxx.xxx.xxx.xxx, server: audiophilestyle.com, request: "GET /<admincp-directory>/?adsess=o1uajgg714eu0hkhgln7vja9i8&app=core&module=settings&controller=general HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.2-fpm.sock:", host: "audiophilestyle.com", referrer: "https://audiophilestyle.com/<admincp-directory>/?adsess=o1uajgg714eu0hkhgln7vja9i8&app=core&module=overview&controller=files"
I checked my config for this site and saw this
# Lock down access to the AdminCP
location ~ ^/<admincp-directory>/.+\.php$ {
try_files $uri @ips404;
include /etc/nginx/includes/php_fastcgi_params;
fastcgi_pass php_backend;
}
I added the two lines in BOLD below and all is good.
# Lock down access to the AdminCP
location ~ ^/<admincp-directory>/.+\.php$ {
try_files $uri @ips404;
include /etc/nginx/includes/php_fastcgi_params;
fastcgi_pass php_backend;
fastcgi_buffers 38 4k;
fastcgi_buffer_size 16k;
}
I think @Makoto may want to adjust his recommendations with version 4.4, for the nginx conf files
https://invisioncommunity.com/forums/topic/424510-setting-up-a-secure-nginxphp-fpm-installation-for-ips4/