I noticed that the checkout was not changing to SSL while testing and is giving errors with PayPal Pro on Sandbox. Also PayPayPro iFrame does not work - I believe that again PP is not seeing the SSL.
I set all configs to https:// for secure and non-secure, If you go to https://store then add product to cart, them go to cart, it reverts to non-ssl http://store Same in admin will not remain in SSL.
I too had inconsistent links; some https and some http when I checked the page source. I solved this problem by editing the system > library > url.php file. On line 18 I added the "s" after http even though it's supposed to be plain http. I know this is a hack; the real problem must be somewhere else but at least I can carry on for now.
version because i think this was fixed ageo ago
Not fixed. I installed a fresh 2.2.0 and the error is still there. The SSL does not work correctly. In fact I am today installing 2.1 to see if I can get a working fresh version with SSL and working PayPal iFrame. Error is related to badly coded SSL calls.