In my case I had to change the back to (which is the default on Ubuntu). Otherwise, the port 443 wasn't using SSL and was sending plain HTML back to the browser.
You can check whether this is your case quite easily: just connect to your server http://www.example.com:443. If you see plain HTML, your Apache is not using SSL on port 443 at all, most probably due to a VirtualHost misconfiguration.
Cheers!
