Authentication problem on checkout

When I try to check out my order in the shopping cart as an anonymous user, I'm redirected to the register / login page. So far so good. When I log in, I'm redirected back to the shopping cart and in the top menu, I can see I'm logged in. However, when I try to checkout again, I'm redirected to the register / login page again while I'm clearly authenticated. Has anyone had this problem before and know how to fix it?
Thank you.

Specify machine key - https://www.nopcommerce.com/boards/t/13269/i-cannot-stay-adminintration-page-ver22.aspx