Cart pages missing after encoding links

The cart page for my products turns into a completely different page with no cart options when I encode the link.

from this:

https://www.jackrabbit.host/whmcs/cart.php?a=add&pid=1

to the encoded version:

https://www.jackrabbit.host/high-speed-hosting/whmcsportal/cart.php/?a=add&pid=1&systpl=portal