Admin Area CSS Blocked Because of Mixed Content

I am having a problem with a website where the front end is completely fine but WordPress’ Admin Dashboard is broken because of mixed content with many console messages of this nature:

Mixed Content: The page at ‘https://www.bchavana.org/admin/’ was loaded over HTTPS, but requested an insecure stylesheet ‘https://www.bchavana.org/admin/css/common.css’. This request has been blocked; the content must be served over HTTPS.
https://www.bchavana.org/:1 Mixed Content: The page at ‘https://www.bchavana.org/admin/’ was loaded over HTTPS, but requested an insecure stylesheet ‘https://www.bchavana.org/admin/css/forms.css’. This request has been blocked; the content must be served over HTTPS.
https://www.bchavana.org/:1 Mixed Content: The page at ‘https://www.bchavana.org/admin/’ was loaded over HTTPS, but requested an insecure stylesheet ‘https://www.bchavana.org/admin/css/admin-menu.css’. This request has been blocked; the content must be served over HTTPS.

I have tried every suggested fix I could find online, including disabling all plugins (except Cloudflare), switching to the default twenty seventeen theme, the solutions at https://manovotny.com/fix-wordpress-admin-styles-not-loading/, etc.

When nothing worked, I installed this plugin. Is there any way I can use this plugin to force the admin side CSS to load via HTTPS? Or is there a way I can do this manually?

Many thanks!
StellaQ