SSL connect error on payment

Whenever the Pay Now button is pressed, all that happens is a message saying SSL connect error. This occurs when using the Guest URL and when logged in from a client user. I’ve setup the merchant in settings using my sandbox API Username, Password, and Signature, with Test Mode enabled. The only thing I’m seeing in the logs is ERROR - 2016-10-18 05:18:39 --> Severity: Notice --> Undefined property: stdClass::$invoice_item_total /invoice/application/views/invoice_templates/public/InvoicePlane_Web.php 193 which doesn’t seem to be related.
Any idea what might be happening?
Thanks in advance.