I'm trying to understand how does Magento generate its HTTP or HTTPS links:
Mixed Content: The page at '...' was loaded over HTTPS, but requested an insecure stylesheet '.../js/calendar/calendar-win2k-1.css'. This request has been blocked; the content must be served over HTTPS.
I can see /calendar-win2k-1.css in quite a few frontend and admin XML files, but where does it read the host from? The database? In this case, I'm used to Joomla / Drupal / Wordpress plugins doing mass search and replace, but I can't seem to find a Magento equivalent.
It was due to a missing configuration for the reverse proxy (h2o) I'm using.
Solved by adding to index.php, before everything else:
$_SERVER['HTTPS'] = 'on';
$_SERVER['SERVER_PORT'] = 443;
I suppose there are more elegant solutions, but whis worked.
This error happens when some of your site resources (like images or css/js files) are being loaded through http protocol while your site is using a https ssl certificate.
Mixed Content Errors is another possible SSL problem mainly due to the failure to load images or scripts via a secure SSL protocol.