I've been looking for hours to get SSL working on my v1.5.2.1 but I can't get it to work. It really drives me crazy

This is what I've done:
* Installed the correct ssl certificates on Apache including the correct chainfiles.
* Checked if SSL is working with http://www.whynopadlock.com. Check is OK, so SSL is working correctly.
* Edited <root>/config.php and <root>/admin/config.php to set the correct HTTPS parameters
* Enabled ssl in the dashboard
I've entered my opencart website and added an article in the cart. It is displayed nicely on the top of the screen. When I hover over the 'checkout' link, it displays https in the statusbar, so that's correct. When I click on it, I somehow get redirected to http and I end up with a message that my cart is empty. Other paged are working correct with SSL (like the my account page). I use the default .htaccess file with nog custom stuff.
Also, I get thrown out of the dashboard when I switch between frontend and backend and I need to login again.
When I disable SSL, there are no problems at al.
I even installed a fresh install of OpenCart so there are no modules active that can cause issues. It has the same result: an empty shopping cart. I had an 1.5.1.3 install that never had any issues regarding SSL.
Anyone else have the same problem with v1.5.2.1?
Thanks.