When you attempt to access the notification URL you get a 403 Forbidden error, and in your PayPal IPN History page you are seeing Retrying - instead of Sent or Failed.
If the IPN shows up in the IPN History list as "Retrying" then this means that communication is being blocked by the server and causing confusion in MemberMouse. You'll want to reach out to your hosting provider to understand why IPNs from PayPal are being blocked, and to have them make your IPN accessible to all web traffic. Verify that there are no security settings on the server preventing access to the IPN file. This will help to insure proper communication flow between MemberMouse and PayPal.