No CSS in multisite subsite admin login when using HTTPS


I've set up a multisite installation with the correct htaccess and wp-config settings. Everything is working fine. HTTPS was used from the start (I didn't change from HTTP to HTTPS).

When I log into one of the subfolder subsites (so, no subdomain) the CSS of the login page is not loaded. The error: "Mixed Content: The page at '' was loaded over HTTPS, but requested an insecure stylesheet ',buttons,forms,l10n,login&ver=4.6.1'. This request has been blocked; the content must be served over HTTPS."

Debugging has led me to "wp-login" which contains the function call "wp_enqueue_style( 'login' );" which in turn calls "wp_styles()", where "_css_href" is the function that generates the (faulty) link to the stylesheet. And that's where I'm stuck. Taking into account that all other links to CSS files are OK, I tend to think there must be something I've missed in the settings or something.

So, to recap:
- multisite installation
- subfolder subsites
- login on main site looks fine
- all CSS on subsites is working: front end, admin pages...
- ...except the login page
- no other issues with HTTPS or malformed URLs

I'm really curious to find out where this problem is coming from. Could you give me some hints?