-1 blank page on return from SagePAy

EXPLANATION OF THE ISSUE

I am experiencing an issue when using WordPress 4.7.2 with WooCommerce Plugin for accepting payment through SagePay Form Gateway. – Version 1.4.1 By PatSaTECH and WooCommerce Version 2.6.14. The page returns -1 instead if redirecting to the order confirmation page.

STEPS TO REPRODUCE THE ISSUE

1. Add product to cart
2. Checkout
3. Complete details and be forwarded to the SagePay page
4. Complete payment and be redirected to the website.
5. Stuck on the api page: https://www.liquorandallsorts.com/wc-api/woocommerce_sagepayform?crypt=@%5Bmassive-guids-here-removed-for-brevity%5D
   Blank white page with -1 at the top left