XAMPP: Call via HTTPS leads to Error 403 (Access Forbidden)

Recently, I set up a small own server in a virtual machine using XAMPP to test my PHP and MySQL projects locally.

This worked surprisingly well and after setting up the virtual domains, I could easily access my websites via the correct URL.

However, with one exception: The pages are only accessible via HTTP, but not via HTTPS (SSL encrypted). So if I'm using a domain like http://www.example.com it is working while you can not access https://www.example.com. I only get the error 403 (Access Forbidden) displayed in the browser together with the note:

You don't have permission to access the requested directory. There is either no index document or the directory is read-protected.

How can that be? There is definitely an index document in the corresponding folder and it is all working with HTTP, too. What can I do?