Generating PDF from HTTPS causes error

Question

I'm using Winnovative PDF Converter to generate PDF files from HTML (URL)

When triggering a button on a HTTP web page, to generate a PDF from a HTTP URL, this works fine.

However when triggering a button on a HTTPS web page, to generate a PDF from a HTTP URL, I get the error:

Could not render the url. Could not get image from url.Navigation timeout..

Why is this happening and how can I prevent it?

Please Note: I'm not getting an error when generating a PDF of a HTTPS page, I'm getting an error when making a web request from a HTTPS page to generate a PDF