Static Front Page https/ssl mixed content errors

Hello,

My SSL is working properly, see for example https://www.hildegardwinkel.nl/page/2/

But any page I select as Static Front Page becomes full of mixed content errors if accessed by https! See https://www.hildegardwinkel.nl

So the weird thing is that when I swap the Static Front Page for a new one, the new one will be bugged instead of the previous one.

Any ideas how to fix that the Static Front Page will display correctly as https?

Thanks!