I'm pretty sure your problem comes from using HTTPS through localhost.

Since all the traffic is contained on the same server between Nginx and Apache, then you don't really need to encrypt that traffic.

And remember to setup Apache to listen on 127.0.0.1 (known as localhost), and just HTTP port 8080. You don't want Apache to listen on your public IP - otherwise there would be multiple ways into your system, which wouldn't be good.

Try this instead in your Nginx php-section:

proxy_pass http://127.0.0.1:8080;

I had a battle with this myself.
Add the following lines to wp-config.php

// If we're behind a proxy server and using HTTPS, we need to alert Wordpress of that fact
// see also http://codex.wordpress.org/Administration_Over_SSL#Using_a_Reverse_Proxy
if (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https') {
    $_SERVER['HTTPS'] = 'on';
}

It should come just before

/* That's all, stop editing! Happy blogging. */

Getting a little closer. But have a different error now. Now I am getting a forbidden error access denied.

Forbidden

You don't have permission to access /index.php on this server.

I have updated the user and group on the directory and files to apache:apache. I have even for testing given them all 777 permissions. Still no luck. Apache is running as user and group apache.

.htaccess file

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L] 

#START - WP Hide & Security Enhancer
#WriteCheckString:1489014293_56908
<IfModule mod_env.c>
SetEnv HTTP_MOD_REWRITE On
</IfModule>

RewriteRule ^modules/(.+) /wp-content/plugins/$1 [L,QSA]

RewriteRule ^assets/(.+) /wp-includes/$1 [L,QSA]

RewriteRule ^site_uploads/(.+) /wp-content/uploads/$1 [L,QSA]
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{REQUEST_FILENAME} -f
RewriteCond %{REQUEST_FILENAME} !wp-activate.php [NC]
RewriteCond %{REQUEST_FILENAME} !wp-cron.php [NC]
RewriteCond %{REQUEST_FILENAME} !wp-signup.php [NC]
RewriteCond %{REQUEST_FILENAME} !wp-comments-post.php [NC]
RewriteCond %{REQUEST_FILENAME} !wp-login.php [NC]
RewriteRule ^wp-([a-z-])+.php /index.php [L]
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteRule ^readme.html /index.php [L]

RewriteRule ^user-input.php /wp-comments-post.php [L,QSA]
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteRule ^license.txt /index.php [L]

RewriteRule ^content/(.+) /wp-content/$1 [L,QSA]

                            <FilesMatch "">
                                <IfModule mod_headers.c>
                                    Header unset X-Powered-By
                                </IfModule>
                            </FilesMatch>

                            <FilesMatch "">
                                <IfModule mod_headers.c>
                                    Header unset X-Pingback
                                </IfModule>
                            </FilesMatch>
#END - WP Hide & Security Enhancer

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

@joshopkins @hansen

By chance, are you using CloudFlare? When you mentioned the too many redirects error, CloudFlare immediately came to mind as that's a very common error, but extremely easy to fix.

With CloudFlare, you need to login and head over to the SSL page, then set the configuration to use Full (Strict). Allow it to resolve and that should fix the issue.

As far as the 400 error, in your NGINX configuration, comment out:

ssl on;

So that it looks like:

#ssl on;

Once that change is made, reload or restart NGINX and let me know if you're still having issues.

@joshopkins

If you'd like to shoot me an e-mail, you can find it in my profile (click on my username). I can make a few suggestions and we can go from there depending on what you'd like to do.

Did you ever get this resolved? After hours of tinkering I have been able to get this working because of 1 simple missing "s".. yes an S... If anyone else is having this issue, I'll post my configs for review... Once I added that s, it worked like a charm.. One issue I am stil having is with .htaccess files.. Apparently the proxy to apache from nginx does not include the ip traffic is originally coming from and this causes deny,allow issues.. I have not resolved this yet but the site is now accessible!! Anything in bold you should probably be changing to your own settings... :)

-------------- NGINX -> ** /etc/nginx/sites-available/domainx.conf**

server {
listen 80;
servername xxxxx.domainx.com;
return 301 https://$host$requesturi;
}

server {
listen 443 ssl;
servername xxxxx.domainx.com;
sslcertificate /etc/nginx/ssl/cert_domainx.crt;
sslcertificatekey /etc/nginx/ssl/cert_domainx.key;
location / {
proxypass https://192.168.0.100:6443;
proxysetheader X-Real-IP $remoteaddr;
proxysetheader X-Forwarded-For $proxyaddxforwardedfor;
proxysetheader X-Forwarded-Proto https;
proxysetheader X-Forwarded-Port 443;
proxysetheader Host $host;
}
}

*-------------- APACHE -> * ports.conf

Listen 8085

<IfModule ssl_module>
Listen 6443
</IfModule>

<IfModule mod_gnutls.c>
Listen 6443
</IfModule>

*-------------- APACHE -> * 000-default.conf

<VirtualHost *:**8085**>
ServerName xxxxx.domainx.com
Redirect permanent / https://xxxxx.domainx.com/
</VirtualHost>

<VirtualHost *:**6443**>
# The ServerName directive sets the request scheme, hostname and port that
# the server uses to identify itself. This is used when creating
# redirection URLs. In the context of virtual hosts, the ServerName
# specifies what hostname must appear in the request's Host: header to
# match this virtual host. For the default virtual host (this file) this
# value is not decisive as it is used as a last resort host regardless.
# However, you must set it for any further virtual host explicitly.
ServerName xxxxx.domainx.com

    ServerAdmin **webmaster@localhost**
    DocumentRoot /var/www/html

    SSLEngine on
    SSLCertificateFile **/etc/apache2/ssl/cert_domainx.crt**
    SSLCertificateKeyFile **/etc/apache2/ssl/cert_domainx.key**
    SSLCACertificateFile **/etc/apache2/ssl/cert_intermediate.crt**

    # Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
    # error, crit, alert, emerg.
    # It is also possible to configure the loglevel for particular
    # modules, e.g.
    #LogLevel info ssl:warn

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

    # For most configuration files from conf-available/, which are
    # enabled or disabled at a global level, it is possible to
    # include a line for only one particular virtual host. For example the
    # following line enables the CGI configuration for this host only
    # after it has been globally disabled with "a2disconf".
    #Include conf-available/serve-cgi-bin.conf

</VirtualHost>

If you append $request_uri to your proxy_pass, e.g. https://127.0.0.1:8080$request_uri; then install mod_rpaf as per https://www.digitalocean.com/community/tutorials/how-to-configure-nginx-as-a-web-server-and-reverse-proxy-for-apache-on-one-ubuntu-18-04-server, then Apache and thus PHP will see the right URL, and you won't need SSL on Apache. You can also tweak the standard WordPress rewrite rules because by the time the request gets to Apache, you already know it's for a .php file.