Checkout works "live", but not "sandbox"

The site is currently set to use PayPal Express Checkout. While the store is set to the "live" setting, clicking on the "continue to checkout" button navigates the browser away from the store site to the PayPal page, to continue editing buyer information such as credit card number, address, etc to complete the purchase. However, in "sandbox" mode clicking on the "continue to checkout" button returns the following error:

"10002 - Security error - Security header is not valid"

and the browser does not navigate away from the store site to PayPal.

The glitch appears to be only while the store is in "sandbox" mode, as setting the store to "live" navigates to PayPal, as expected. I wish to be able to test the payment gateway without having to make a payment, as I am building this site for a customer. The site is online now. If you wish to do so, you may run a test yourself. The site is currently is "sandbox" mode.