IPN request could not be verified by PayPal

I have a function that adds credits to a users account when they purchase credits via a Gravity Forms form. This has been working fine until a transaction came in today where the payment status hasn't changed from Processing and there is no transaction information.

I enabled the logging addon and resent the IPN and get the following error

DEBUG --> Response: WP_Error Object
(
    [errors] => Array
        (
            [http_request_failed] => Array
                (
                    [0] => couldn't connect to host
                )

        )

    [error_data] => Array
        (
        )

)

ERROR --> IPN request could not be verified by PayPal. Aborting.

Any idea what could be causing this?