Paypal Instant Payment Notification (IPN) settings

my site has been up and running on ver. 1.54 for years, today we received an order and we never seen it in the Opencart dashboard & never received an email. I check around and seen that some people were having issues with the Instant Payment Notification (IPN) settings. So I went to PayPal and went into the Instant Payment Notification (IPN) settings and try to enable it again and now I get the following error:
We were unable to validate the URL you have entered. Please check your entry and try again.

According to PayPal support the said this;
I've tried using the IPN URL http://aquariusfountainsnmore.com/index ... d/callback and add it into my account but unfortunately not worked. So you have to reach Opencart technical support team to check whether the page was existed in your website server or not and also available to be connect.

Any assistance would be greatly appreciated.

Regards

John