Magento 2.2.5: Admin login page redirect loop 301/302 after installing SSL and enabling HTTPS

Preconditions

1. Ubuntu 18.04 DigitalOcean droplet
2. Nginx 1.1.4
3. MariaDB 10.1.34 (MySQL 15.1)
4. PHP 7.1 (Using FPM)
5. Redis server 4.0.11

Steps to reproduce

1. Download Magento 2.2.5 via Composer
2. Install Magento 2.2.5 via CLI using existing database
3. Install SSL certificate.
   (At this point the website is functioning properly).
4. Enable https on magento frontend and admin.
5. Execute setup:upgrade,setup:di:compile,indexer:reindex,cache:flush and setup:static-content:deploy in that order.

Expected result

1. Frontend is displayed without any security warnings.
2. Admin is displayed without any security warnings.

Actual result

1. Frontend is displayed without any security warnings.
2. Admin is stuck in 301/302 redirect loop
3. I can see in the headers that the SSL certificate is recognised as valid.

Notes

I have this problem running 2.2.5 with nginx and redis, no varnish, all on a single digitalocean droplet.

I have read that it can be to do with cookie configuration and set cookie_path to “/” and cookie_domain to both “.example.com” and “example.com”, neither of which have produced successful results. I have made changes to the default nginx config provided by magento as suggested above but that wasn’t successful either. The nginx config file for my website is as follows:

upstream fastcgi_backend {
         server  unix:/run/php/php7.1-fpm.sock;
}

server {
        listen 80 default_server;
        listen [::]:80 default_server;
        index  index.php index.html index.htm;
        server_name  example.com www.example.com;

        set $MAGE_ROOT /[web_root]/magento;
        set $MAGE_MODE production;

        access_log [path_to_access].log;
        error_log [path_to_error].log;

        include /[web_root]/magento/nginx.conf.sample;

        return 301 https://$server_name$request_uri;
}

server {
        listen 443 ssl;
        index  index.php index.html index.htm;
        server_name example.com www.example.com

        ssl_certificate /path/to/ssl.crt;
        ssl_certificate_key /path/to/ssl.key;
        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
        ssl_ciphers [ssl_cipher];
        ssl_session_cache    shared:SSL:10m;
        ssl_session_timeout 24h;
        keepalive_timeout 300s;

        set $MAGE_ROOT /[web_root]/magento;
        set $MAGE_MODE production;

        access_log /path/to/access.log;
        error_log /path/to/error.log;

        include /[web_root]/magento/nginx.conf.sample;
}

Edit

I raised this as an issue on github and was advised that I should set the unsecure base url to https, then remove the 301 redirect from my nginx config. I’ve done this, restarted the server and re-deployed magento to ensure everything is completely fresh. Deleted browser cache and disabled caching in code inspector prior to visiting website and admin page still returns a redirect loop.

Edit #2

I looked at app/etc/env.php and noticed that, alongside the usual base, media, static etc. entries that are found within the system>web>… entry in env.php, there was another entry at the bottom of the file with just the base urls, both set to http.

Although they are duplicate entries, I didn’t want to delete them because I’m not sure why magento has created them (I’ve never manually edited the env.php file for this instance). Instead, I just updated every value which began “http://” with “https://…”.

Now I can access the admin panel, but the configuration options for secure URLs in Stores>Configuration>General>Web>Url Options are greyed out and the frontpage no longer redirects to https, though there is no mention of http anywhere in the config file.

I guess that it is greyed out because I’m running in production mode, but I want to make as few changes as possible to the system because it’s so close to functioning.