In the links that you posted you are going to the same pages, but there is an access restriction in effect, the URLs are showing you what page you tried to access and were restricted on.
cart.php?&checkout=1&accesscheck=%2Fcart.php%3F
So it seems that you have an access restriction rule on your checkout page or the cart page that is preventing you from getting there. If this is the case then you will need to login before you can get to these pages. If you do not want to restrict access on these pages then you will need to remove the access restriction rule from them by going to Modify > SiteAssist access page manager.