"Request exceeded the limit of 10 internal redirects due to probable configuration error" while having WordPress behind a reverse-proxy

I've read dozens of questions/answers and blog posts but none worked for me. To give you some context, what I want to do is:

• have my Rails app at example.com running on Heroku
• have Wordpress at example.com/blog running on DigitalOcean

I tried a lot of different things but in the end I couldn’t get through this error that appears when I try to access a blog post (accessing the blog homepage or the admin works fine):

[Tue Mar 30 21:46:24.047355 2021] [ssl:info] [pid 818] [client 54.89.204.173:53088] AH01964: Connection to child 2 established (server blog.example.com:443)
[Tue Mar 30 21:46:24.047582 2021] [ssl:debug] [pid 818] ssl_engine_kernel.c(2372): [client 54.89.204.173:53088] AH02043: SSL virtual host for servername blog.example.com found
[Tue Mar 30 21:46:24.057128 2021] [ssl:debug] [pid 818] ssl_engine_kernel.c(2254): [client 54.89.204.173:53088] AH02041: Protocol: TLSv1.3, Cipher: TLS_AES_256_GCM_SHA384 (256/256 bits)
[Tue Mar 30 21:46:24.057482 2021] [ssl:debug] [pid 818] ssl_engine_kernel.c(415): [client 54.89.204.173:53088] AH02034: Initial (No.1) HTTPS request received for child 2 (server blog.example.com:443), referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057636 2021] [authz_core:debug] [pid 818] mod_authz_core.c(817): [client 54.89.204.173:53088] AH01626: authorization result of Require all granted: granted, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057645 2021] [authz_core:debug] [pid 818] mod_authz_core.c(817): [client 54.89.204.173:53088] AH01626: authorization result of <RequireAny>: granted, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057879 2021] [core:error] [pid 818] [client 54.89.204.173:53088] AH00124: Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace., referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057885 2021] [core:debug] [pid 818] core.c(3832): [client 54.89.204.173:53088] AH00121: r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057888 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057891 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057894 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057896 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057899 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057902 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057904 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057907 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057911 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /blog/index.php, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.057917 2021] [core:debug] [pid 818] core.c(3838): [client 54.89.204.173:53088] AH00122: redirected from r->uri = /hello-world/, referer: https://example.com/blog/
[Tue Mar 30 21:46:24.058076 2021] [ssl:debug] [pid 818] ssl_engine_io.c(1102): [client 54.89.204.173:53088] AH02001: Connection closed to child 2 with standard shutdown (server blog.example.com:443)

Here is what I’m using and their settings

I’m using rack-reverse-proxy gem on the rails app:

config.middleware.insert(0, Rack::ReverseProxy) do
  reverse_proxy_options preserve_host: true
  reverse_proxy /^\/blog(\/.*)$/, 'https://example.com$1'
end

On WordPress general settings I have “https://example.com/blog” on both WordPress address and Site address fields.

My .htaccess is the one created by WordPress:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /blog/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>

Virtual host in /etc/apache2/sites-enabled/000-default.conf:

UseCanonicalName On

<VirtualHost *:80>
        ServerAdmin webmaster@localhost

        ServerName blog.example.com
        ServerAlias www.blog.example.com

        DocumentRoot /var/www/html

        <Directory /var/www/html/>
            Options FollowSymLinks
            AllowOverride All
            Require all granted
        </Directory>

        ErrorLog ${APACHE_LOG_DIR}/error.log
        CustomLog ${APACHE_LOG_DIR}/access.log combined
RewriteEngine on
RewriteCond %{SERVER_NAME} =www.blog.example.com [OR]
RewriteCond %{SERVER_NAME} =blog.example.com
RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]
</VirtualHost>

More clues

Something that might be a clue: when I access example.com/blog/wp-admin without being logged in, it redirects me to https://example.com/blog/wp-login.php?redirect_to=https%3A%2F%2Fblog.example.com%2Fwp-admin%2F&reauth=1. And I have no idea where this "blog.example.com" domain comes from.

I have the "blog.example.com" subdomain added to the WordPress droplet on DigitalOcean's droplet settings.