Paypal sandbox fatal failure

Hi,
I've been trying to set-up the paypal sandbox with Hikashop.
I set the paypal url to api.sandbox.paypal.com/nvp,and the paypal email address to my sandbox login email, but I get a white page with "Fatal Failure" when the cart sends me to the paypal sandbox.

Any idea's?

thanks
Chris

The sandbox URL is www.sandbox.paypal.com/cgi-bin/webscr
That's why it's not working.

Thanks Nicolas!