Theme preview is blank (because https is not supported)

Hi. The theme customizer preview appears blank. I detected that in my case the option FORCE_SSL_ADMIN and/or having the /wp-admin/ folder with https is what causes the problem (browser blocks mixed content). If I configure the site to use http only it works fine.

There’s some URL cooking in the preview code that forgets to keep the used protocol. I would like to use https (because I want to use certificates to get to wp-admin). Where should I look in the code? I’ve been looking into wp-admin/customize.php but I didn’t understand the code very well.