On kolab.org/blogs/, some assets are linked to using http:// rather than https:// (for example, [[ https://blogs.fsfe.org/greve/files/2015/08/Kolab-Now-fb-cover-image.jpg | this image ]] from blogs.fsfe.org.
This causes a severe warning in visitor's browsers and the visitor may in fact also get alerted, and/or the content may get blocked