We're running a multi-domain Magento store with 2 domains,
1.) domainA and domainB
2.) When checkout, we move the customers to domainB with the session in the link.
3.) both domain is running over SSL. (https:// ... )
Everything worked perfectly in 1.8.x , but after upgrading to 1.9.1 the session is lost when redirect the customer to domainB.
If the SSL if OFF in domainB - the session is working perfectly...
but if SSL is ON, the session is lost in domainB and the cart is 'empty'.