SSL?

Does either your backend or frontend already has SSL enabled?

The backend is SSL only, the frontend offers both SSL and non-SSL.

Unfortunately, WordPress forces you to use a hardcoded site URL and since the site is not SSL only yet (mostly due to caching issues), I have to use a hardcoded https:// URL.

I see, you can use the ppp_preview_link filter to replace http with https in the preview link:

function ds_https_preview_link( $link ) {
	return str_replace( 'http:', 'https:', $link );
}
add_filter( 'ppp_preview_link', 'ds_https_preview_link' );

Cool, that did it, thanks!

(It would be great if the plugin would detect https:// by default, however, the issue itself is resolved for me. Thanks for your timely response!)

@chmars It does, but only if the home URL is https.