Need help with IPN forwarder

I am using PayPal for a multi site with Pro Sites and for another single installation using Membership 2. Using this thread I tried setting up an IPN forwarder
https://premium.wpmudev.org/forums/topic/multiples-ipn-dynamically-setting-the-notification-url

However I just received a notice from PayPal that IPNs sent to the url I set up are failing. I have support access granted on the site where the forwarder is set up. Could you please look into this & let me know what I need to correct? Both these sites are live & I'm in a bind with this. Thank you!