Some files are loaded from https while others are loaded from http.

HTTPS detection is incorrect, especially you get this with a default NGINX
setup.

In the file:
\lib\classes\class.cms_config.php

There is many times this written:
if( isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] != 'off' )

In NGINX the $_SERVER['HTTPS'] is always set, but if it is not https, it will be
empty.
The assumption of the code is there for incorrect, it should be something like:
if( isset($_SERVER['HTTPS']) && !empty($_SERVER['HTTPS']) && $_SERVER['HTTPS']
!= 'off' )

A quick fix for NGINX user is currently to do this:
Edit your:
/etc/nginx/fastcgi_params

There is this line:
fastcgi_param HTTPS $https;

Replace it with this:
fastcgi_param HTTPS $https if_not_empty;