How to Fix 404 Error Message on a Subdomain When Accessed Via HTTPS
The most common cause for this is that the SSL certificate you have does not cover the www for your subdomain.
To fix this, go to your Client Area -> Websites and open Site Tools for your website. Go to Security -> SSL Manager, select your subdomain from the Select Domain dropdown and choose Let’s Encrypt in the Select SSL field. Click the Get button and allow a few moments for the new SSL to be issued. After that your subdomain will be accessible via https.