Nginx – SSL termination Nginx + Backend Nginx not working (get mixed content result in HTTPS)

I set up 2 nginx webserver from this tutorial https://www.digitalocean.com/community/tutorials/how-to-set-up-nginx-load-balancing-with-ssl-termination

The problem is, a simple one test php page via SSL is loaded nicely, but when I try to install some PHP Application like Moodle, I got mixed content warning and the UI is broken.. (some in HTTP mode, some in HTTPS mode, etc…)

How can I get all content loaded in all HTTPS (fix the mixed content thing)?

Here is the frontend SSL Nginx config:

# File: \etc\nginx\sites-available\main.big.vm

upstream mainBigVm {
    server main.big.vm:80;
}

server {
    listen 80;

    listen 443 ssl;
    ssl on;
    ssl_certificate         /etc/nginx/cert.crt;
    ssl_certificate_key     /etc/nginx/cert.key;

    server_name main.big.vm;

    location / {
        proxy_pass http://mainBigVm;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

And here is the backend Nginx config (in server main.big.vm):

# File: \etc\nginx\sites-available\default

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    root /var/www/html;

    index index.html index.htm index.nginx-debian.html index.php;

    server_name _;

    location / {
        try_files $uri $uri/ =404;
    }

    location ~ \.php$ {
        fastcgi_pass unix:/var/run/php5217-fpm.sock;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }
}

update 170430-1

I've tried the suggested config in front-end, but still not work.

upstream mainBigVm {
    server main.big.vm:80;
}

#suggestion
server {
    listen 80;
    location / {
        return 301 https://$host$request_uri;
    }
}

server {
    listen 80;

    listen 443 ssl;
    ssl on;
    ssl_certificate         /etc/nginx/cert.crt;
    ssl_certificate_key     /etc/nginx/cert.key;

    server_name main.big.vm;

    location / {
        proxy_pass http://mainBigVm;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

update 170501-1

I noticed a strange behavior too. If I type the HTTPS URL with ending slash, the URL is loaded, but if I type without the ending slash, somehow the URL is converted to HTTP with auto added end-slash