Hi guyz, I am a fan of Hika, and had the same problem as OP, whenever I was trying to proceed to checkout it showed the message "cart empty" and that was only after I changed to the sadbox url above that this was happening, while with the normal url it worked fine.
So the solution for me was it had to be a "URL" and from nicolas' answer above the "http://" part is missing, so when I added the protocol prefix it solved my problem. Thought my suggestion would help somebody.
TL;DR
The sandbox url should be
http://www.sandbox.paypal.com/cgi-bin/webscr
Note:- just noticed that the forum is hiding the http part so edited and added code tags.