After doing some research on this, it sounds like this is likely due to a configuration issue, and there are a few things you can try.

You can check your Wordpress config file (wp-config.php) and try adding the following lines of text (replacing example.com with your domain):

define('WP_HOME','http://example.com');
define('WP_SITEURL','http://example.com');

This can also occur due to a misconfiguration of a particular plugin. You can try disabling plugins one at a time to see if any individual plugins have been corrupted or are outdated.

Additionally, there is another Community post where a user had a similar issue with Cloudflare and their SSL certificate. In the post, one of the solutions mentioned was to change the SSL settings to "Full" rather than "Flexible". I'm not an SSL expert by any means, but this definitely might be word a shot.

I hope this helps you get started with troubleshooting or at least points you in the right direction!

I have the same problem. I have 2 sites on subdirectories:

https://awplife.com/docs/ (normal WordPress site)
https://awplife.com/demo/ (WordPress multisite)

Thanks to @jdutton, The DOCS site working now with solution, but Demo site not working.

My demo site is WordPress Multisite.

here the .htaccess file code of DEMO site:

#awplife.com/demo/

# BEGIN WordPress
RewriteEngine On
RewriteBase /demo/
RewriteRule ^index.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
# END WordPress

Hey, @awordpresslife - Glad I could help a little bit! I'm sorry it didn't work for the Demo site. If you happen to figure out the issue in the meantime, definitely post another reply. I'm curious what the issue is myself!

DEMO is the directory name of WordPress multisite.

Thanks