If you see an error like this after completing a test transaction or live transaction with Paypal with your OptimizeMember site, you’ll need to ensure that your site is using an SSL certificate. This is a recent change on the side of Paypal as they recently started requiring all new IPN connections to use HTTPS in the URL. 
The reason you are likely getting this error is if you recently setup the Paypal IPN for the first time since Paypal had made this change. OptimizeMember IPN URL is now set as https so when you copy that URL from “OptimizeMember > Paypal Options > Paypal IPN integration” it already has the https in it, so what is causing this error is if you do not have an SSL and your site is currently operating under HTTP protocol instead of HTTPS. Essentially the server is not able to verify the identity of the site due to this issue.
We suggest asking your host to install an SSL for you (they may charge for it). If your host is compatible with Open SSL then they may be able to offer that to you, or you may consider using CloudFlare.com flexible SSL as that is good for the IPN at Paypal, however if you are using Paypal Pro and taking credit card data on your site you should consider the best protection and go with a full SSL option.
We have two guides on changing over to SSL:
Use something other than the CloudFlare SSL
If you are already using an SSL and see the above error, then please check the following:
If you already have an SSL and still see this issue, you will want to make sure to go back through the Paypal Integra tion Guide to be sure nothing was missed and also check to be sure Paypal IPN/PDT settings are turned on in Paypal.
You may also wish to install our Server Sca nner tool which can help identify any server incompatibilities which may need to be addressed with your web host in order to fix the issue.
You may also check this guide for furth er details or contact our support team using the link below for help.