Opencart checkout link/button redirects to cart when ssl is switched on

I'm having this problem:

Now I have this issue

1) user clicks on check out the link -> the page: /shop/index.php?route=checkout/checkout begins to load using the https: security.

2) after about 1.5 to 2 seconds later the page suddenly redirects back to /shop/index.php?route=checkout/cart using http:

If I turn off the SSL option in admin->settings->server, the checkout page loads up fine and everything works.

But with the SSL on, it gets aborted and then the browser redirects to the unsecured cart page.

the config files are correctly set up for ssl, and there doesn't seem to be any problems with the ssl certificat, so I don't know what's causing the problem.