Block Editor : Mixed Content Error : Can’t save posts

I am currently running 5.2.2 of WordPress behind a reverse proxy. When editing a post using the block editor, we get the following errors in the browser console when trying to update a post or save a draft:

—
Mixed Content: The page at ‘https://www.mysite.com/blog/wp-admin/post.php?post=1234&action=edit’ was loaded over HTTPS, but requested an insecure resource ‘http://www.mysite.com/blog/wp-admin/post.php?post=1234&action=edit&message=4’. This request has been blocked; the content must be served over HTTPS.

Uncaught (in promise) TypeError: Failed to fetch
—

The errors are NOT present when we use the Twenty Nineteen theme AND disable all of our plugins. When ever we activate any of the plugins we use
OR change our theme we get the mixed content warning above when editing posts using the Block Editor. The Classic Editor seems to work fine.

We have the following customizations in our wp-config.php:

—
if ($_SERVER[‘HTTP_X_FORWARDED_PROTO’] === ‘https’) {
$_SERVER[‘HTTPS’] = ‘1’;
define(‘FORCE_SSL_ADMIN’, true);
}

if (isset($_SERVER[‘HTTP_X_FORWARDED_HOST’])) {
$_SERVER[‘HTTP_HOST’] = $_SERVER[‘HTTP_X_FORWARDED_HOST’];
}

define(‘WP_SITEURL’, ‘https://www.mysite.com/blog’);
define(‘WP_HOME’, ‘https://www.mysite.com/blog’);
—

Our plugins are up to date and include:

– AddToAny Share Buttons
– Akismet Anti-Spam
– Classic Editor
– Contact Form 7
– Insert Headers and Footers
– Jetpack by WordPress.com
– List.ly
– Qzzr Shortcode Plugin
– Simple Tags
– Yoast SEO

Theme: Salient

Any thoughts on what may be causing this issue and how to fix it?