I've changed the URL of our site from beta.example.net to example.net and now CiviCRM is loading with the Drupal frontend theme instead of it's own.

I've enforced the change by updating the line in .htaccess that's enforcing https. It now reads as

RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://example.net/$1 [R=301,L]

I've also updated the base URL in Drupal's settings.php to

 $base_url = 'https://example.net';

and the site URL in civicrm.settings.php with

if (!defined('CIVICRM_UF_BASEURL')) {
  define( 'CIVICRM_UF_BASEURL'      , 'https://example.net/');
}

I've also tried setting the CMS root in civicrm.settings.php with both

define('CIVICRM_CMSDIR', 'https://example.net/');

and

define('CIVICRM_CMSDIR', '/var/sites/c/example/public_html/');

but neither seems to make any difference.

My best guess was that I'd broken something with Shoreditch, trying at some point in the past to define a relative URL so that CiviCRM wouldn't constantly display an error. But I've gone to https://example.net/civicrm/admin/extensions (I can't navigate to it through the current theme) and disabled Shoreditch and the error is still occurring.

Drupal's own backend loads as expected (though somewhat slowly). Drupal 7.67. CiviCRM 5.18.2. Shoreditch 0.1-alpha34.