This is a post from René Breedveld, who experimented this error and wanted to share his experience and solution. I am very thankful with him for taking the time to write this.
I’m using the Archaius theme with great pleasure till the moment I changed my Moodle site form http to https. Every time I opened the https site, a pop-up message warned me about unsafe content. Looking for the solution I used https://www.whynopadlock.com/. Their report pointed out that Open Sans fonts, within the Archaius theme, were causing these insecure messages. But after extensive digging I couldn’t find any php code responsible for this.
So I had to decide between taking these insecure messages for granted or using another theme. I decided for the first option.
As a result of not yet executed lay-out issues I updated my logo for the site and the mobile version. This turned out to be a great move! After the adjustments made, the pop-up about safety risks disappeared. So I came to the conclusion that these images were blocking the full SSL access due to the shift from http to https. In short: Archaius is 100% SSL safe and I’m still using it with even more pleasure than before.