Have reviewed your install. It's failing because CBSubs can't reach PayPal Sandbox. A 400 error is being thrown. Are outbound connections to PayPal being blocked by your install? If you've any sort of firewall please try disabling it or contact PayPal regarding what IP Addresses you need to whitelist. See CBSubs > Notifications where it specifically informs you of a communication error. CBSubs will not pay a subscription unless it can verify the payment.
Your issue sounds like exactly the same as the below. I recommend contacting PayPal to ensure your account is ready for testing (completely active).
www.joomlapolis.com/forum/255-developer-members-support/234503-cbsubs-paypal-pro-gateway#285152